A larger fin would help your Rio L to plane earlier, with the centerboard stowed up inside the board, but
a 5.0 m2 sail is very much too small for a sailor your size/weight on a board as large as the Rio L unless you are an absolute beginner, or you are sailing only in => 25 knot winds.
To get you Rio L to plane, and work as a oversize high volume shortboard you would need a larger rig for more normal conditions.
If the winds are 12-16 knots, then an 8.5-9.5 m2 rig would seem about right.
If your winds are in the 15-20 knot range then a 6.5-7.5 m2 rig would probably be what you need.
The 5.0 m2 rig is just too smalll to get a big guy and a big board going fast enough to unlock the
performance aspects of your Rio L.
As far as fin designs, if you need a weed fin, the a large one 40-52 cm would be good (with a larger rig).
The short "cleaver" style fin you Rio L came with works pretty well with the centerboard deployed, but is very much too small for you with the centerboard stowed and a big sail.
You could get a larger vertical fin like a 52-60 cm slalom or race fin and use it to some advantage if you do not have weeds/shallow water to contend with.
If you could suggest what sort of fin designs you are thinking about, we can certainly help you select something that will work well in your Rio L.
There are just too many fin shapes/ designs to cover them all.... wave fins (not good for the Rio), small slalom and free race fins (also not good for the Rio)... cleaver type shallow water fins, (OK for the Rio with the CB down, not so good with the CB up) Free race fins in the 52-62 cm range (what you need to be looking for if no weeds); 40-52 cm weed fins ( very good if you have significant weeds, pretty good even without any weeds.
Hope this helps,