K.I.S.S. ...not that the last "S" applies to you.
The lower the flow through the sump...the easier it is to keep bubbles from returning to the display. They bubble up before being swept forward by currents while still rising.
Small return area is relative...but I would go on the small side for ATO sensitvity. However, calculate your return/drain lines and go from there. If you're worried about draining the return section upon start-up...then initially let the overall water level in the sump be above the single baffle separating the return section. As the water starts pumping you'll see how much more water you can remove from the system so that each compartment has it's own water level when the return pump is on. Just try to keep the skimmer cup out of the water at all times...pre- and post-activation of the return pump.
Get yourself in the right ballpark with the sump baffle heights and don't be afraid to redo the plumbing, and pipe height, where you dump water into your sump. PVC is relatively cheap as this hobby goes.
Have you looked at Melev's website and his sump designs? Good place to start IMO