Roger Williams University in Rhode Island is one option to consider. If you want to go a bit further north and quite a bit further east, Acadia University in Nova Scotia is very good for both environmental sciences and biology, and has a marine biology concentration that is available for biology majors. It is reasonably priced for international students (note the exchange rate), and is located right on the Bay of Fundy (or at least is twice a day at high tide). Both Roger Williams and Acadia have just over 4,000 students so the size might be about right.
UNH has a good marine biology program. UVM is good for environmental sciences (one daughter was briefly an environmental sciences major there until she changed her major). Both are however bigger than it sounds like you are looking for.
I also agree with considering the College of the Atlantic, and the University of Maine.