and she has no problem with it nor do I. That being said, she is 5' and I'm 5'2".
A friends wife is 5'6" and she has trouble but that can be due to her only kayaking once a year. In some instances, it's easier for women to kayak than some men as we tend to move with our hips (and our weight is centered there) but men's weight is more in their chest area.
But the inability to stay upright could be an individual trait as there are many males that paddle these SOT's and have no problem.
Your best bet is to try one out. Give it a try and best wishes.
Gedi Convertible Helmet