Oh, normally I'd prefer to keep the characters as accurate to their original design's as possible.
Also I'm trying to keep Mona's relative size and proportions relative to how she was portrayed in Wario Ware with Wario (As well as Ashley and Kat & Ana I suppose) who she isn't
too much taller than them from what I'm seeing, so I don't think I would want her to tower over them and all, height wise. (I think making her about a head shorter than Peach should work out fine, at least looking at the height differences between Wario and Peach and Wario and herself, she's also a young adult after all, at least, if I recall correctly)
Though then again, Mona's design seems to be pretty inconsistent throughout the Wario Ware series and she seems to have gotten a more proportionate design in later entries in the series so I guess I could try maybe hitting a middle ground of some sorts for her....
Edit: Just had a quick check through in both Smash and Wario Ware. If Mona where to appear in Smash (Be it playable, Assist Trophy or anything else) I think she'd be roughly around this tall.
I think they would want to keep her relatively accurate to the rest of the Wario Ware cast height wise, which seems to be around this scale (Not fully accurate to scale mind)