In terms of numbers 200+ is what a lot of boys school have such as Kings, Hampton, Whitgift etc so that's not in question, that is possible and can be done to a very good standard. Large form entries can work.
However I would be asking about how the site will cope, why the big bulge this year as opposed to other years, is this a permanent change, will they be securing more space and how do they prevent a parent feeling like they've diluted their offering when on paper it looks like the school is staying exactly the same but with many more feepayers coming in. I'd be asking about extra investment in facilities, teachers, support staff and opportunities and how they will ensure each and every girl is looked after and provided for to the same level that current girls are.