I'll have to measure my 5.7 rudders when I get out to the boat this weekend.
But I have two spare rudder blades at home, for my 5.5 and I just measured them at 35" long.
From manuals, the 5.2/5.5/5.8 & 6.0 all have the same rudders.
So, I don't know about the 31" rudders, that seems real short.

My 5.7 rudders are longer than my 5.5 rudders, but mainly they are shaped different, as azcat mentions, they "jut" forward underneath the casting. This rudder shape allows it to feel more balanced, even when it is heavily loaded. i.e. even when heavily loaded from the side, it can feel more neutral since more of the area is closer, or even in front of the pivot line.

The main issue I have with 5.7 rudders is indeed the length. It would be great if the rudders were longer. Issue is that pointing a 5.7 involves putting nose down, to reduce transom turbulence at minimum, but more so to use long pointy hull to its advantage.
The long hull length (compared to 5.0 which supposedly has same rudder length) combined with the "height" of the rudder casting above the waterline(skeg + hull shape compared to 5.2/5.5) means that sometimes very little area of rudder is in the water. Sometimes this area is too small, and it can stall out.

I have seen 5.7 (and 5.0) with both kinds of rudders - plain "round" rudders, and I've seen 5.0 & 5.7 with rudders of kind with shape that juts forward. I have a 1987 model 5.7, it has style that juts underneath. Pretty sure they are original, I bought from original owner.

Jim Casto