If you have a super wide (100.5 cm wide) formula type board with the footstraps > 40 cm off center on each side, you can control a 70 cm fin pretty well, even in high winds and at high speeds.
When the fin size gets too big, and the footstrap offset does not provide enough leverage to really control the fin's attitude, you get the dreaded "tail walking" (can happen with smaller fins at extreme speeds also).
The board tends to come up off the water slightly and oscillate rail to rail.
You will only understand this phenomenon by experiencing it once or twice I can assure you.
You simply lose the ability to keep the fin at a controllable attiude.
You normally simply change down to a smaller fin with perhaps a less powerful foil profile and things
are good again, even at hgher speeds.
If you want to "experience" tail walking and all it's scary perturbations, get someone with a 40 mph motor boat to give you a tow. Stay inside the wake and at some speed, your fin will reach it's critical control speed and your board will indeed "tailwalk" and dump you in the water at fairly high speed.
The big 70 cm (it's in the rules that 70 cm is the max.) formula fins, on 100 cm wide formula boards, with some of the best formula sailors in the world, can go up around 30 knots in some pretty knarly conditions with 9-11 m2 sails before they get to the tailwalking stage.
On narrower boards, big fins become "out of control" at much lower speeds.
All of the WS speed records have been set with really tiny fins. I have a set of 7"-9.5" True Ames Speed
Serious fins that are wonderful...if you are going fast enough.
But you have to sail a bit off the wind and stay off the fin when getting going as they will spin out in a heartbeat at slower speeds. Get up over 25-30 knots and they are solid as a rock.
P.S. The 58 cm should work nicely in your Rio M!