Maybe you should wait until your bass spawn and then start removing some. That way you should get some new genes in your pond before culling.
I'm not sure about Pa. but here in Ms. bass spawn usually in April and May. So, I wouldn't think you have to wait too much longer. As far as size goes it would be a guess. I think I would opt for the 8"-10" size removal, And tend to hope the ones that size that you stocked had grown larger.