I've measured Speedy G at least a half-a-dozen times since I got him as a three-year-old. He's a bit taller than when he first arrived, but I think he's done growing now. In my experiences, Arabians can grow until they're about 8 years old. Speedy is 7 and a 1/2, so I'll measure him again in the spring to see if his withers have come up any more. For the last year or so, he's come in at a solid 15'1.
Although I keep saying I'm going to get one, I don't have one of those handy measuring sticks. Instead of the stick, I use a long builder's level and a metal tape measure. Before you try this at home, it's a good idea to introduce the level and metal tape to your horse before you start whacking him in the leg with a noisy yellow snake. I'm just saying ...
Anyway, to measure my horses, I make sure they're on a level surface, preferably mats on cement. I ask them to stand as square as possible. Once they're standing quietly and are comfortable with the tape measure and level, I pull out 65 inches on the tape measure and engage the little locking mechanism. I slide the metal end-clip of the tape measure under my boot. I then hold the tape as close to the horse as I can while still keeping it plumb. I lay the builder's level across the highest point of their withers. If the level is long enough, it will reach the tape measure. When the level is ... level, I read where it touches the tape measure. I divide the inches by four. Speedy G is 61 inches tall - 15 hands, 1 inch. Sydney is exactly 64 inches tall - 16 hands even.
At different times in the shoeing cycle, Sydney is probably taller, as is Speedy G. Both boys had their feet done on Friday. If I had measured them before the farrier came, they might both be an inch taller. I like to measure immediately after the farrier comes because they have the shortest hoof wall they'll have, but the height of the shoe is now added. I figure that the fresh trim evens out the new shoes. So if you want a taller horse, measure him in shoes the day before the farrier comes!
Speedy G is a much smaller horse than Sydney. He's smaller everywhere. His chest is narrower, his legs are thinner, his neck is thinner, and his body is much shorter from front to back. His neck is longer however, and his withers are far more pronounced than Sydney's are. Even so, I was surprised that only three inches separate my boys. I knew Speedy G's measurement was accurate as it hasn't changed in over a year. I measured Sydney three times to verify that he was only 16 hands. He feels much larger.
So to set the record straight, Sydney's little bio over there on the side has been changed to reflect his current height of 16 hands even. Have fun measuring your own ponies!