The prayer grip is when your palms are exactly facing one another, with your thumbs level on the front of the grip which facilitates level shoulders at address. Two Thumb produces grips that are designed with the prayer grip in mind. For example, Superstroke offers a grip designed specifically for the claw method, with the geometry and taper of the grip built to fit your hands more comfortably in that style. The reason that it is important to make a decision on this first, is that grip manufacturers now offer specific grips for different methods. Are you going to use a standard, right below left, reverse overlap grip? Or are you going to be using something different? There are so many ways to grip a putter claw grip, left below right, prayer grip, the list goes on.
Firstly, you need to decide how you would like to position your hands on this new grip because this could have a significant impact on the best design and size of the grip you choose.