How big will it be? Digging the pond is the easy part, moving the dirt is where it takes all the time. If you don't get discouraged and are willing to spend hundreds of hours on your tractor going back and forth several thousand times, maybe more, then it's very doable.
Things to consider.
Depth of the pond. The deeper it is, the more water it will hold and the less likely that it will dry up.
The steeper the bank, the quicker it gets deep, the fewer weeds that will grow along the shoreline.
You really need at least four feet of water to keep it a pond. Anything less and you have a marsh. Not that a marsh is a bad thing, but for resale and aesthetics, open water is the most attractive.
When spreading the dirt, do it in shallow lifts for compaction. Don't just dump the dirt and keep piling more dirt on top of it. spread it out over a large area and very slowly build it up. Every time you drive over it, try to create fresh tracks in areas that you have not driven before. This will compact the soil real nicely and give you a solid dam, levy or hill.
If this sounds like more then you were expecting, figure out what it would cost you to run your tractor for ten hours in fuel. Multiply that by how much dirt you can dig and move in that amount of time and figure out how much you need to move. My guess is that it's going to be allot of money in just fuel because you will only be able to move a small amount of dirt at a time.
I dug a 3/4 acre pond with my full sized loader backhoe. It took me two months, almost 7 days a week to dig it down 4 to five feet with two areas that were 8 ft deep. I gave up on trying to put the dirt where I wanted it and just dumped it along the side of the pond to beat the rains. Then I spent the rest of the winter and part of the spring getting rid of that dirt pile. My bucket holds just over a yard of dirt, and I spent close to a thousand hours doing it this way.
When you get an idea of how much money you will spend, a clue on how many hours it might take, triple that number. This is what it will cost you to do it.
Now consider what it would cost to have it done with a D6 sized dozer. Don't mess around with anything smaller, they are too slow and cost just as much money to hire. A D6 sized dozer is just under 40,000 pounds and with the right blade, can push 6 yards of dirt at a time. What he can do in a week will take you all year. A bigger dozer would be faster, but they jump in price and take special permits just to move them from job to job.
Pond Boss Magazine Home Page! is a good site for more info, but most of those guys are more into the fish part of the ponds and not building them. Some have tractors and do some of their own work, and one of the moderators, Otto, is a contractor the builds ponds for a living here in Texas.
HeavyEquiptmentForums.Com is a good site for bigger tractors and the professionals who operate them. Some of the guys there dig ponds all the time and are very helpful in what it takes to get it done, the proper technique and tool for the job.
No job is too big to tackle on your own.
Good luck,
Eddie