I would say that providing the saddle fits correctly and you don't do too long a session then as you're not very heavy then you should be ok on the flat. The other option of course would be to find a competent child to help out!
I would think youd be ok, have you got a pic of you both be easier to judge.
I have this problem, have a sec a 11.2 and im 5'11 sp defo to big, i use either very small teenagers or i do lots of long reining find this really helps them and alot more than lunging.
