Regular length will fit an S-Bar. Or they do so on a 2018 mode, earlier models may be slightly different. The Gripshift variant GP series has a shorter grip. Too short for comfort on a S-bar I think. Remember the S/L sizing on GP grips refers to the outer circumference of the grips - aimed at hand comfort. The bar diameters are identical, as is the grip length.
I have GP3s regular size on my S-bar with no modifications, no need to move the brake levers inboard and no issues with the fold.