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...