Micro pig is just a marketing name. I personally have never seen a min pig under 75# The norm is 100 to 150# and by definition anything under 350# is considered mini.
Your best bet is to visit and see parents. You'll want to see them when they are at least 5 years old. You need to make sure it is a very respectable breeder and not someone who bought a pair and threw them together. You'll also want a vet who works with pigs who lives reasonably close.
For quality info look here.
http://americanminipigassociation.com/