i like it... looks nice....you might want your baffles a little taller to allow you more volume in the return area. you can always add something under your pump to lessen the maximum volume allowed to go to the display tank in case your drain plugs.
the reason some people make their return area small is the fact that if something plugs your overflow, your return pump will run dry before it floods your house.
evaporation rate is about a gallon a day... i top mine off about every 3 days.
As for building your return area bigger....it really depends on how much volume you can put into your DT without it flooding. To figure that out, find out where the water level is in your DT during normal...
heres an image of my sump i built using a 55g. The fuge is half of the tank to take advantage of the center brace and my 24" light fixture. If you notice, there is an extra panel after the fuge with a slot open about 2 inches ff the bottom... this allows for any sediment to fall into that area...
you can also do some math and figure out how much extra volume your DT can handle if your overflow plugs. Then make sure the volume of your return pump area does not exceed that amount.
easiest and best way..... remove top brace...... take razorblade and scrape off all silicon.... no need to take panes apart.... dont remove the silicon between the panes either... leave it.... once you get it all scraped off, take some acetone on a rag and thoroughly clean all seams..... get...
here is my original plan but after much research and ideas from many people, i came up with what you see below it. it works really well and you can adjust the flow to your fuge and your skimmer.