Pond management?

[mako1]

My in-laws have an 8 acre pond that was dug out for hwy 71 many years ago. I've been fishing it for bout 2 1/2 yrs. biggest fish I have caught is bout 6lb last year and 1 bout 5 this year. Just wondering if anyone knows tips to which ones to throw back and ones to keep. It is full of bream and white perch. I keep anything from maybe 1 1/2 to 3-3 1/2lb.nothing over 4 for sure. On good days ill catch 10-15 . Today I went out and flipped a jig and caught prolly 15 or so biggest bout 4. Any suggestions to grow big ones.