I would normally buy cob size for my thoroughbred. What size would fit best please.

This would depend on the size of the horse and the length and width of cannon bone

Medium -  would be 14.2hh-16hh with a cannon length of 25cm and a width of 20-22cm

Med/Wide -  would be 14.2hh-16.hh with a cannon length of 25cm and a width of 22-24cm

Large - would be 16hh-16.3hh with a cannon length of 27cm and a width of 22-24cm