Yes, use 45 mil. We have a 1000 gal pond, 8 years old. This next spring
going to double or triple the size. Used 45 mil last time and will again.

PS. Private pond nice feature. There may be insurance/liability
implications if one 'builds' a pond and if someone were to wander in
(especially if property unfenced) and fall in. This similar to a
having an unfenced, locked gate access, private swimming pool on one's
property. If pond is/was a natural feature of the landscape the
situation might be different?

It's fenced, 6' stockade and I'm armed. Nobody wanders in.

To the OP, make it bigger than you "think" you want.