Did you mean the Mobile Suit or the Pilot? If the Pilot - just make sure you go to a Super-Tailor. Usually the one at the back of the Icon to the right or the one in the Tiki club in Pocket D. And when you open the super tailor, you get the initial screen that lets you change from Male, to Female to Huge and also change the height slider.
If you mean - how is the Robot suit THAT BIG (like about a head taller than the top of the usual scale)? Well you see - there's this little editing trick I know of... 😄