If you are replacing the counters choose sinks you like the hole is cut for those sinks not sur what your issue is.

Since you're replacing the countertops it doesn't matter what size the new sinks are since the new countertops will have holes cut for them. If you like the current size, look for something similar since you can't find anything exactly the same size. Pay attention also to depth. When I remodeled my primary bath a couple years ago I got sinks which are on the small side, but they're deep, which I like. Smaller sinks mean more counter space. However, if you're putting new sinks and countertops on older vanities which aren't as tall as modern ones, make sure the sinks aren't going to be too deep for the existing plumbing hook ups. If you do have old, shorter vanities, I suggest you consider replacing them as well unless you have shorter members of your household who prefer them. Since you will have to disconnect and reconnect the plumbing, now is a good time for that.