With the thick trees in the watershed, you should not have an issue with siltation, unless there's a lot of construction going on in the watershed. Don't really see how leaves would be a problem though.
The recommendation he gave you is very non-specific. The amount of water you'll get off a 20 acre watershed will vary greatly depending on several different variables. Those variables include average watershed slope, vegetation type, vegetation condition, soil type (sand =percolation, while clay = more runoff), and of course the intensity of the rainfall event. FYI the trees will help you by slowing the water as it comes down the watershed, moreso than open pasture.
I think you will be fine with the 0.75 acre pond. Just make sure you have a wide enough spillway (enough to handle a big storm) and you compact the dam well during construction. Recommend packing in 9" layered lifts as a minimum. If necessary, a sheeps-foot roller helps compaction if the soil has a high clay content.
