Get a brush effect bigger than 2500px?

[drw158]

Hello, I like to make my own brushes and such but I need to cover a 10" by 10" 300dpi canvas with a single brush. Currently, the brushes cannot go larger than 2500px. So how can I get the brush effect (removes all white pixels to transparent)?

I make brushes out of scans from old paper, so there is no need to move the brush once I click.

I have tried messing with the layers modes, but none seem to do the trick. I have also bitmaped the brush material (old paper scans), with a diffusion dither to convert it into a halftone image in order to place a color onto the bitmap, but I'd rather not do that. I'd rather keep the material greyscale and not convert it into black and white pixels.

thanks!

 

[drw158]

More info

here is a link to an example of my work. As you can see there is a slight paper texture in the black background. I used a brush, but since this is a 10" by 10" image 300dpi, a brush cannot fully encompass the whole canvas. What you see is a bitmapped and dithered greyscale paper texture in the background ( i don't want to dither it. I want it to be fully greyscale but in another color besides black)
photo link