Hogue makes the smallest SP101 grip there is, but it's still surprisingly usable for what it is. It's way better than the garbage that comes on the SP101.
I don't have a good recommendation for you, as I searched for years to find a solution. I've owned pretty much every SP101 grip you can get, and not one of them was very big. Technically the biggest, most hand filling grip out there is the Lyman Pachmayer, and I like them on other handguns, but the angle on the SP101 is so bizarre. It's like you have to break your wrist all the way down to a nearly uncomfortable level. I think Ruger is missing by a mile on what the SP101 could be a a light field gun, and in 357 magnum or 327 federal with 4" barrel it can be a fine hunting handgun on it's own, but without the grip, it's just not that great. All of the grips out there are orientated for concealed carry and are quite small. There is no full sized field grip out there for an sp101. Most of them are awkward. The Hogue is both one of the smallest, and one of my favorites.
I even called some custom grip makers. Harretts would not do it when I checked, but they are my go-to, they are great. One guy I called basically laughed at me saying it's a tiny gun, it needs a tiny grip. Yet the LCR, a smaller and much lighter revolver comes with a bigger grip, and aftermarket has bigger grips than anything you can get for an sp101. The Hogue tamer for the LCR is a great grip, its really, really stupid they don't make that exact same thing for the sp101.
Unfortunately what I did was sell my SP101's. I had two with a 3" barrel, and one with the 4". One was even a custom 3" with fully adjustable sights, but without a good grip they will never be that fun. The LCRx is better than the SP101 for shorter barrels, and if you want the longer barrel, the GP100 is all around a better handgun, with the exception of being bigger and heavier of course. Another option is the single six, which if you are ok with a single action, they are great shooters.