Your description is a little vague as to what the problem was, but I'll take a stab at what seems to have gone wrong.
You glued up your cross bracing and after it dried and you took the clamps off the top and back ended up bending like a potato chip (the wrong way). If this is the case then you braced up in too high a humidity and then when the humidity dropped the top and back lost some moisture and shrunk across the grain (as wood does). It's not the size of the brace, but careful attention to your RH that you need to be paying attention to.
Also, if this is what has happened and the RH stayed pretty much unchanged from when you braced it, then got the instrument body closed up without any drama's, then the RH dropped back to normal levels, or much lower you face the very real probability that the top and back would concave and possibly split.
If you want to do this for a living, a serious hobby, or just build quality instruments that aren't going to self destruct because of being built in the wrong RH range, then you absolutely must get a good, accurate and reliable hygrometer and either some sort of control of your building environment, or only build when the readings are in the right range.
When I do a commission that I know is going to an area of the country that has a radically different average RH to what is the norm (in Australia it can get very, very dry in some areas in the centre) then I'll build to a much lower RH than one that will live near the coast.
For what it's worth, my soprano's use 6X6mm for the transverse braces.