If I'm understanding your question...I don't think you need to use a brush for that. I don't use PS, but in PSP you can just draw a box. In PSP there are a bunch of shapes you can draw including a rectangle, I'm sure there's something like that in PS.

If you need more specific directions, maybe you could try posting in the PS thread. But maybe this is enough info to start you looking in the right place?

hth

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jordan in cinci