A 300 ft long dam is average. My project is about 5 ac and the dam will come in around 325' long, about 15 feet high at the center. There are guys on this forum that have dams in the 500 - 1000' long range.
WRT the trees, you have to make a decision. Trees and dams cannot occupy the same space...period. If you want to leave them within the pond basin, this if fine. It will create structure for the fish and a natural appearance for the pond. Make sure, tho, that this is asesthic appearance that you are looking for. Now is the time to get ALL this stuff rite.
It sounds like you are working with a standard impoundment pond, requiring only a suitable dam. This is good news. The rule of thumb in pushing dirt is "less distance, the better". Ideally, your dam soil should come from within the pond, nearest the dam. Assuming you have the clay soils required for proper dam construction, you will want to push it out of the basin in front of the dam and create some depth for your pond. Based on your geographic location, you are gonna get some hard freezing action. In order to create a zone for proper fish protection, 8' deep may be an issue. There are others on this forum that will be able to kick in the guidance about proper winter depth and DO levels.
Make no mistake. If you can create a 5 ac pond with a 300' long dam, you will NOT opt to create an excavated pond instead. If all your stars line up (proper soil, springs that will not reverse, proper construction technique), build the dam; the water will come.
