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Depends on what you're using it for. A render that needs a nice high quality texture, low or high poly game. If the wall is going to be put next to other walls seamlessly. Whether or not it's a basic clean/new brick wall, or will it have some grunge to it.

Also what else is in the scene if it is for a game. Like 1 wall, or 50 walls and buildings and 100 players...etc...etc.........so many variables will give different answers.

Basic will let you get away with smaller, 512x512 textures. For some more detail in the wall, you might wanna go higher 1024x1024, or 2048x2048.

The rest of the work will be in the uv mapping. The whole 1 side of the wall piece doesn't necessarily have to take up the entire texture size. Break it up into 2 or 3 sections and get a nice seamless texture going and you'll up the quality of the wall, without going with a bigger texture.