"It is one size too small for me."
Translation:Tá sé méid amháin ró-bheag dom.
and the other question is:- Why is this "Tá sí méid amháin ró-bheag dom." wrong
Ró is always used with a dash before a word that starts with a vowel and it generally doesn't use a dash before a word that starts with a consonant, but it does often enough. (Ró-mhaith, ró-chontúirteach, ró-leitheadach mar shampla.) I agree with you regarding beag, though.
I was just going by the standards given in the TEG B2 syllabus, which always showed ró- without the dash before consonants (though it didn't show any starting with <f> so I can see this one might not be, since it lenites)
Would Tá sé aon mhéad amháin ríó-bheag dom be right as well?