DVD

I sold my Bianchi 19' MTB because I felt that it was too big for me.
I am 5'6'' tall. Should I get a 15.5' or a 17.5' frame??

MtnBikerChk

Hard to tell on paper - except that a 15.5" would probably be way too small.

go try them and have someone fit you that knows what they are doing.

MeHT

I'm 5'9" and I need 17.5"-18". The frame size depends on your legs, mostly. When you stand over the frame you'll want about 2" clearance from the top tube.

Maelstrom 

Depends on the riding. For pure xc it may be too small. But for any out of bounds or freeriding you may do that should be fine. I ride a 19" at 6'5 and for more and bigger tricks I use a 17"

DVD

Thanks for the advice. It'll have to be 15.5 as the 17.5 frame leaves very little clearance bet my crotch and the top tube.

Kustomz

There is a real nice sizing system at Wrench Science;
https://www.wrenchscience.com/WS1/default.asp