They're not, like.....super huge or anything. Here's a rough example of what the characters look like inside the mech:
View attachment 295
So they're not that big. That said, although it's technically a viable and accurate option, I wouldn't be a huge fan of it, since when you get down to it Sakura Wars is such a character-focused series that it'd be a little lame if the main character was just stuck inside a mech for the entire fight, unable to show any of her personality, mannerisms, etc. It'd feel weird for a series where such a huge part of the appeal lies on the cast of characters. At least IMO.