the 8k x 8k option is only for loading a texture file, not for editing ( i should add a lock for this)
the problem is the bitmap itself. 8096 x 8096 x 6 bit (for the colors) 402.653.184 bit + file stuff mean you have a texture of at least 384mb uncompressed in the ram which is much to big to handle for the tb in the paint mode.
if you want to use this size of texture (i can't imagiton why you need this on a car, even my 220m heavy cruiser only uses 8192 x 4096) then you have to use an external grafic tool e.g. gimp, ps or corel