Well if you can get them as brushes won't that make them less huge?  I think 2500x2500 is the largest you can make a brush, but once it is a brush then the .abr file isn't huge, is it?  Not even with many brushes in it...? 

I'm wondering if this tut on making brushes from patterns (it includes how to make the pattern in the first place but you won't need that part) could be of any use to you?

http://www.eyesontutorials.com/articles/2873/1/How-to-make-a-brush-and-how-to-use-it/Page1.html

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