dont make things harder than they need to be.
:)

be sure the wall scale is equal to the scale of you picture..for instance, if your picture is 4 x 3...then  your wall needs to have the same scale dimensions.  So a 640 x 480 wall could have anysize image that of the same scale dimensions (like 640 x 480, 1280 x 960, etc)

now if you really want to be technical
assume your wall is 8 feet tall, then you can get 8 feet of bricks in there at one whack..and a larger resolution of the same scale image will help you get closer without pixelation.  So a NTSC video shot of 720 by 480 could easily stand a texture of 1280 x 960 because this lets you get closer to the wall without losing detail.

at any rate, guestimate (or just measure your brick) and use the absolute measurement of brick bottom to brick bottom (including the mortar) for each brick and then divide the wall by that height.

anyhow, dont make it harder than it needs to be.
go grab a few picts, make a wall of the same scale and then go from there.

luck
BT