Hello. I was thinking about offering some new and interesting mats here on Renderosity but before I do I have a few questions...

1- What physical size? 4" x 4" or what would be best
2- What file size?
3- What resolution?

Thank you all for your help!
gonedigital...

I think standard sizes like 512, 1024, 2048 etc (I think thats right) would be good. Checkout the resolution sizes in the TERRAIN EDITOR and go from there. 
You might want to think about providing a range of sizes to meet differing needs of the users.
As for Physical sizes, I've no idea. 

Hope this helps.

Available on Amazon for the Kindle E-Reader

All the Woes of a World by Jonathan Icknield aka The Bryster

And in my final hours - I would cling rather to the tattooed hand of kindness - than the unblemished hand of hate...

 Also Base 8 dimensions are also more efficient and render a tiny bit faster. I'd say 2048, user could downsize it for less "Hi Rez" use.
Just my opinion but I'd say max file size of around 1 to 2MB.
You mean as in DPI? Then I'd guess 72 dpi - I don't think Bryce uses any more then that or converts the size back to 72.