If there were certain patterns in two different (opposite?) parts of the CMB, that would suggest it wraps. None have been found, so if it does wrap it's bigger than that.
I mean, there’s plenty of evidence for the Big Bang, and if that’s the case then the most likely shape that falls out of the maths is a hypersphere, like inflating a balloon as others have said.
Which one it is depends on the density. There is a critical density at which the universe is flat, and our current best measurements basically allow for all three possibilities. It's just hard to say if something is flat or actually spherical but really, really huge.
