Hello, Kurt, and welcome to the Forum!

 Quote:
1. I was hoping to build a 2-3 acre but am concerned about water supply/watershed. Using a topo map or anything else, how do calculate pond size?

Legitimate concern - pond size supportable by runoff depends on factors including watershed size, rainfall, evaporation loss, leakage, and watershed cover type (think forest vs. grassland vs. parking lot).
a. See the local NRCS office. If you're lucky they have the time and experience to go over the topo map with you and figure this out.
b. Post that topo map and provide other info as available. We have some excellent topo readers here who can help figure out pond size.
c. Hire a pond consultant.

 Quote:
2. Since I am new to all of this, is it easier to build a smaller pond say 1 acre and then expand later?

This is perfectly feasible and has been done more than once. You would have to move more dirt (and spend more) for an expandable 1 acre pond than for a fixed one acre pond, if you want to raise the dam later and increase it's size. There may be other routes to do this, but that's the way I'm familiar with via Bob Lusk's stories in "Perfect Pond ... Want One?" (Buy a copy - best $35 you'll spend on your new pond!)

 Quote:
3. Can anyone recommend a dirt person in Green County, Wisconsin? I did get a list from the County but no one the
recommended.

Open for Cheesehead Pondmeisters ...

 Quote:
I can not take possession of tha land until the corn is harvested in November so I have all winter to read and plan.

I recommend getting ground cover (grasses) planted ASAP after corn harvest, to cut erosion on you new land.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
[Linked Image from i.pinimg.com]