if you have more liner then I suggest going as large as the liner allows (with in reason of course ...dont cut yourself short!) seems everyone always wishes they went bigger fter the pond is done.
for a pump think about an out of pond pump, in the long term much more cost effecient and easier to work with. dealing with an in pond pump is a pain in the tush!
the folks here are wondeful and will happily answer questions and chat ponding /plumbing with you. Steve knows his stuff! you wont find better people anywhere