Size of a return duct
A 10&quot;x 10&quot; register box would be fine with an 8&quot; flex duct.
I think you need to do a bit more research into your AC system though. 8&quot; flex duct has a lot of resistance to air flow, much more than rigid duct. Depending on how well your existing system is balanced, supply and return, you might find out that adding that register won't help get rid of that trapped hot air. If you connect a length of 8&quot; flex to a return plenum or another rigid return duct, the resistance of the flex might be too much to make any difference. Have you considered just adding a ceiling fan to move the trapped air around so the existing system can remove and cool that air down?