The simple, traditional method of height estimation is to measure the length of the cannon bone. That length, in inches, will approximate height at full maturity (presuming proper husbandry over time).
There are other, more complex formulas. They might be more accurate, but have a lot more steps. This one meets the "close enough for government work" standard.
To deal with a "bowl spiller" get a 4'x4' piece of heavy plywood, a black rubber bowl, and three heavy duty bolts, nuts, and washers (6). Drill holes in the center of the board and match holes you drill in the bowl. Attach the bowl to the board using the bolts, with a washer on each side. Now put it in the feed area, fill it, and stand back. The horse will likely try and lift it, but it won't work. The bowl is attached the board and the horse is standing on the board (that's why we use a bigger piece of wood
Use the heavy, black rubber bowls as they approach indestructability. I'd stay with a very flat one, as that gives the horse the least "purchase" when they attempt to pick it up.
This vice should "go away" reasonably quickly, but stay with it for several months (or until the plywood rots) to ensure the undersired behavior has been suppressed.
Good luck with the horse.