Valves come in various sizes, where 1/2" and 3/4" are the most commonly available. A 1/2" supply line can source about 5-6gpm safely. A 3/4" valve over double that. If your supply lines aren't larger all the way, a larger valve will have little effect (some, though) on the available flow. A pressure balanced valve will have no effect on the flow unless one side's supply pressure drops, then it compensates to try to retain the balance of hot/cold, which means slowing the flow to the side that still has higher pressure. The internal design of each valve differs, so look at its design flow in gpm, then choose one that meets your needs. Under normal conditions, it shouldn't reduce the flow below that figure (but note, that figure is based on your water pressure at a certain point, flow will change some as it either increases or decreases from the test value).
Like anything mechanical, it will eventually need service, so don't put it burried behind a tiled wall unless you have access in some manner that doesn't destroy your tiled wall.