In my experience, it's pretty hard to go too big, but it's possible. I made an external filter and got a pump that was too big and the water pushed the top off the filter and it flowed all over the floor!

What kind of filter are you using? If it's in the pond, and your turtles are too big to be sucked in by the pump, then it won't be too big.
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0.1.0 ornate box turtle
2.2.0 eastern box turtles
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.9.0 chickens
1.0.0 Dutch(rabbit)
3.2.0 adult children
1.0.0 husband