64 x 64 pixels it is, you must also remeber to watch the file size. I think it can be no larger than either 6k or 8k on the file size........
res ( resolution ) will be what determines how big a file it is when it is set at 64 x 64 pixels 64 res, 68 res, 72 res ( most use 72 resolution I do believe )
to get a good idea of what your limits are is to go into the squads sub-folder and look at the properties of some of the nosearts uploaded by other squads...
look at resolution , look at file size, and look at what color bit they are set to, bit meaning 8 bit, 16 bit, 256 etc........
It would be a waste of time for you to make a nice looking squad patch / nose art and then have it no be accepted......
hope this helps.