Love them or hate them, Giant Robots are a part of a genre that a lot of gamers are always going to love and are firmly established in the roots of gaming history. Movies like Pacific Rim, animes like Gundam and Macross, table top games like Battletech, and computer games like Mech Warrior have the genre healthy and all around us. However, before you go dropping giant robots into your world there are some things you want to consider and have ready to answer. Otherwise...well, things get kind of silly.
Question 1: Why Giant Robots?
This is the first and biggest question you need to answer. It is also one that I feel many series with giant robots fail to answer adequately or properly. Still, the question is there: why giant robots? What need has prompted the militaries of the world to make giant robotic humanoids (or at least legged/armed tanks) as the primary means for fighting each other?
In the movie Pacific Rim and in many Gundams this question isn't answered. This leaves the audience, when they think about it, feeling a bit disjointed because you have to come to terms that someone basically made a decision to take this approach - which is generally not super efficient in my opinion - and others went with it. For deeper analysis, in Gundam the mobile suits are meant to be fighting in space, so why the humanoid form? Now that question can be answered with what purpose the suit has in the story (it hides the full horror of war behind a robot while still letting you show what is happening) but for space combat there is nothing particular about the classic humanoid form that makes it stand out as the ideal form. A sphere with guns would probably work a lot better, in all honesty.
By contrast, Robotech (Macross: Super Dimensional Fortress) has this question answered with the Zentraedi. See, humanity knows that the aliens are 50 foot tall humanoids. That means when fighting them humanity has to be ready to take on 50 foot tall infantry. Now, in ground combat and in other operations, there are a lot of strengths that an infantryman can bring and a lot of versatility that can be brought to the table as well. The answer for humanity was the Veritech Fighter (and the Destroids as well, but they aren't featured as much.) The Veritechs can fight as 50' tall infantry, but can also be a vehicle for more efficient and adaptable combat.
Because of these factors, the veritechs and destroids of Robotech feel a lot more natural in the world than the Gundams and Zekus of the Gundam verse. Robotech had a specific problem, giant people, and so giant robot people is a sound answer. Gundam just had a lot of people randomly decide that the giant robot was the weapon platform of the future and just went with it. Pacific Rim now makes sense only if you accept that at some point humanity decided the best, and only, way to fight Kaiju was to have a giant man punch it in the head repeatedly.
Question 2: Why Do The Giant Robots Look Like They Do
Form needs to fit function. I kind of addressed this when talking about Gundam and Robotech above and in a sense they go hand in hand. Why are giant robots the answer? Because they provide X function which is the answer to the majority of the problems presented in one go. So why do the giant robots look like they do? Now you can answer this with the line I started with, or you can come up with some other reason as to why it is that way.
For example, Mech Warrior and Battletech giant robots are basically walking tanks. They don't really have hands (some do) as much as they have mounted weapon cannons. They move around on legs, but beyond that fact they basically just work like giant super tanks. So why do they have the legs instead of treads? Because legs fit their mobility needs better in a number of ways. First off, while treads/wheels may be faster on flat terrain they have problems on uneven terrain. Legs don't have this problem. With legs you can handle unstable terrain easily, and keep things moving. You can also brace - increasing stability - in multiple directions. The primary reason here though would be the expected un-evenness of the terrain. Especially since once the fight starts, you know things are going to get heavily cratered. Heavy Gear solved this same problem by giving the mechs a 'road mode' where on flat terrain they could use wheeled/treaded transport but in uneven terrain they shifted back to legs.
So an example of the other reason? I'm going to go back to Pacific Rim. At it's core the human form doesn't seem the best way to fight the Kaiju. The Kaiju come from the ocean, and we know from our own experience that the human body isn't the greatest way to fight in water. However, considering the neural interface used in Pacific Rim it is entirely possible that the human frame is there so that the brain can pilot the robot more efficiently. After all, the human brain has been driving a giant humanoid shaped thing all its life. It also makes the "matching the control scheme to what the pilot is doing" a lot easier.
Question 3: Do You Have Hands? If So, What For?
The two above are the giant questions, but this is one I want you to think of wile you are considering why the giant robots look like they do. Does the robot have hands? If so, why? Hands serve a purpose. The opposable thumb on the human hand is a big part of the reason that humanity climbed to the top of the foot chain. How did we do that? With tools and weapons. Hands let you grasp things, so if your giant robot has hands why would you send them out there unarmed?
Fact is, if the robots primary purpose is to punch things, you'll have a lot better impact with a solid object rather than a hand. If the weapons are all mounted in/on the robot and don't need to be grasped then why have the hands there? In world this is a very serious design question. The hands are complex and they are vulnerable. By using hands when you don't need them you open up vulnerabilities to the robot.
So what do hands do well then? Well, like I said, grasping tools. Giving the robot hands is a great way to making them more versatile in their deployment. You need a blade to cut something? Give the giant robot a sword. you need a long range shot taken? Give it a rifle. Medium ranged combat with lots of enemies? Give it assault rifles and SMGs. Now, these all could easily be built into the machine, but then you have problems with space. Make it so the robot grips them and you now have a very versatile fighting machine.
This method is employed in Robotech, though not with the versatility you'd want, and even better in Gundam and Heavy Gear. Fact is we've been making various guns for human sized people forever. So, why not upscale those and now instead of having to build a different kind of mech for each approach, you just build the same mech and re-equip it for the job at hand.