It's really hard for me to believe that you would fit over a 49cm frame but not a 50. I think that ideally, you want standover, but, it doesn't have to be an exact thing, just enough that you can straddle it with your feet on the ground. You straddle a frame with your pubic bone, not your boys.

Also, when I hear 49 cm, I immediately think of a European frame measured centre to centre. That's actually about the same or a little taller than a 50 cm frame measured centre to top.