Practice pads for drumming come in all shapes and sizes, and the size you should get depends on what applications you’re going to use it for. If you want to place it on top of your snare drum, I’d suggest getting one that is between 10” and 14”. It will fit quite snuggly on there, and playing it on the snare will give you the added response of snare wires. If you want to simply place the pad on a coffee table when playing, you could get a smaller one that is more affordable. Some drummers love playing on small pads as it improves their accuracy.