What is the Max FOWLR tank that anyone has been able to keep successfully with only a skimmer and with out a refugium and sump, 75 Gal ? I believe I am only able to keep my 50Gal FOWLR with just a skimmer; because of it’s relatively small size.
If you provide adequate amounts of rock and skimming, IMO there's no real limit. As with any other system, you just need to balance the bioload/import with filtration/export.
Sumps/refugiums are nice to have and can be a nice asset, but IME they're certainly not required to have a successful system.