i own a QH cross with TB and Arab she's 14.2hh when my instructor measured her it came out to be 78 when i bought the 78 blanket it looked too big then i bought a 75 and it looks better but i could still go down a size.
Measure him. Start the tape at the front of his chest, then wrap it around one side to the tail. The blanket measurements come in inches, usually in increments of two. If it's an odd number, round up, not down.