I have no doubt that there are women who can surpass the vast majority of above average men (womens' ultimate fighting has more than proved that), but I think their number is too small to count on them showing up in any numbers in a military force, so that would be more useful if we were trying to get a Roman Joan of Arc rather than a female legion.I know I've read something on it somewhere, but I can't remember. It's enough to matter - you might find some women who can keep up with the above average men, but it would be very small.
I think "equal to the average man" might be easier than 5%, but "able to serve as medium/heavy infantry or cavalry" would be smaller. Maybe not quite as bad in some other societies - like the Scyhtians - but Roman women, even Roman commoners, don't have a lot of experience with being expected to literally do the heavy lifting, so nature and nurture reinforce each other.
5% is just an educated guess, since I doubt reliable statistics exist for a subject as obscure as the number of women who are stronger than the average man. As for the bit about nature and nurture, true, gender stereotypes could definitely cause Roman women to be weaker than their Scythian counterparts. I suppose that just makes female legionnaires les likely, even compared to female soldiers in other militaries at the time.