I've been changing mine around a lot recently. On some maps, typically dark ones I like a small crosshair (1800) and other bright ones I need a bigger crosshair (default or bigger). Mostly for visibility.
I use 10000 too. I find that it helps alot on long shots, aiming is much easier. The only downside is that it's hard to estimate recoil on the AK. But that doesn't bother me much, I use the mp5 anyway.
I believe the max is 4000. I have mine set to 9999 just to be safe.
I like using yellow. It is visible against the majority of surfaces.
Other colors are difficult to see against blue. I find I have the most difficulty aiming when I have to adjust on the vertical axis, so something that is always visible on the sky/ground gives me the best capability.
If done properly this will cycle your crosshair colors from dark blue to bright yellow when you move right/left or fire, and I find scale 4000 to be a good choice on all maps, not so big it distracts from your target and not soo small you have issues seeing it......