2.0 sized shock will use a 2.5″ID coilspring. A 2.5 sized shock will use a 3.0″ID coilspring. The spring lengths can vary, but in most cases you’ll use at least twice the length in springs as your shock travel, up to an additional 2″ beyond that. So an 8″ travel shock will need to use at least 16″ of spring, up to 18″ total. That could be done in dualrate (6-8″ spring over a 10″ spring) or single rate spring of 16-18″. Any coilover 10″ travel or longer will need to use 2 springs per shock.