The ribs fitted to most sporters seem to be about 10mm (or a bit over 3/8in) wide, and parallel.
This indicates it is the preferred width and configuration for most people.
However, most over-under game guns seem to have 6mm (1/4in) ribs, and some are tapered.
I may be missing something here, but I have never been able to see why there is a difference, accepting that the sporting disciplines are supposed to be a simulation of field shooting.
Really, there is no set rule, and it all comes down to personal preference.
I recall one very successful sporting shooter who had his gun fitted with an inch-wide rib – yet most shooters would find that totally unacceptable.
Personally, I prefer the 10mm parallel rib, to the point that I prefer to shoot game with my Sporter.