You're looking at things completely backwards... What is the GPH / processing capacity of your skimmer? There's no reason to send more water through a small sump like that on a small tank if your skimmer isn't even going to be able to process it all. Don't use your sump return to make up for lack of water flow in the tank from closed loops / powerheads. In regards to the overflow box / return pump specifically: You can have too large an overflow box gallon per hour capacity in relation to your return pump without any real issues (it'll only flow as much water as the pump can sent through it) but if you get too large a pump you'll have a guaranteed flood. If I was you and using one of those siphon-based overflow tubes I would use a very small return pump. At 10x flow through your sump (which might even be too much since it's a small trickle filter and I don't see your skimmer in it...) you only want 500 gph coming from your return, and your existing siphon box is probably more than adequate for that purpose as long as you keep it clean (clear overflow tubes grow algae and clog QUICKLY!).
Ideally? Toss the trickle filter / bio ball chamber nitrate factory and turn the thing into a normal sump if at all possible. Put your skimmer, heater, etc down there to get it out of the the main tank, and don't send more than 10x your total SUMP volume (NOT 10x tank volume! If it's a 20 gallon size sump with only 10 gallons of water in it, then 100-150 gallon per hour maximum through it) or you risk having a bunch of problems with microbubbles unless you put in bubble traps before the return pump. You can probably push it to 20x SUMP volume if you really want to for some reason, but again you risk having a bunch of microbubbles everywhere depending on how you have things set up.
