Welcome fellow NY-er.
First off as a primer, get the book from this site: Perfect Pond, Want one?
Second: Do a lot of reading here, tons to learn of great advice!
Third: I believe your pond is not too small. You will need to plan ahead for cover, depth, etc.
Fourth: If you plan a diversion ditch to prevent runoff from the field reaching your pond, you should be fine. That is of course if that isn't your only source of water for filling the pond. Manure will lead to all sorts of difficulties, least of which is an algae-choked pond.
Fifth: Test holes before you dig for viable sealing clay. The holes you have dug that hold water may not be good enough. They may be only revealing the current water table which can fluctuate much more than you are willing to tolerate. A well-sealed clay-lined pond is by FAR a much more attractive option than a ground water pond.
Get the book first, go from there. Your best move has been to come here! Lots of folks here came after they had a pond dug and are trying to fix mistakes such as leaky ponds. I came here after I had my pond dug, and really wish I found it first!I would have made a few more important choices that are too late to make now.
