Dimensions would be a bit misleading, as they very pretty significantly based on what pieces are attached, or how full you stuff it, or how much you crank down on the compression straps. You can certainly put fins in that are a bit longer than the main bag itself!
However, if you want approximate dimensions, I'm sure Dive Caddy could supply those, or I could measure the panels on my main bag when I get home from work.
FWIW, I usually detach the top bag and have little difficulty fitting it into the overhead compartment in the appropriate orientation, with Mares Avanti Quattro fins.