The classic formula is 1/3 of the oar length inboard and 2/3 outboard of the locks. Rowers with a powerful stroke (and/or an easily driven hull, which a raft ain't) often go considerably longer. I like to keep at least 4 inches clear between the grips— figure out how much space you need between the grips with the oars straight out and subtract it.
Given 54" lock to lock, -4" thumb clearance, that's 50" x 0.5 = 25" inboard with 50" outboard, or 75" (that is 6'3") which seems pretty short. But I started out with 6.5 ft. Sawyer wood utility oars on my 12 ft/16-inch cat and ran that way for two years. Main complaint was the steep angle to the water— the blades tend to dive in hydraulics.
So I just went to 225 cm Gull oars (7'4") with fairly narrow blades.
The feeling of blade heaviness might be heavy blades
, too wide a blade, or having too much length outboard. Different oars feel, well, different. I tried 7.5 ft. Carlisles and they felt blade-heavy and clunky. The Gulls (@1 lb. lighter with smaller shafts and slimmer blades) feel about right.
Shap's right— what's the distance between the oarlocks? That's the starting point.