Trailhorserider, you know I have never thought to try the string test on any of my adult horses. That would be an interesting test. I'll do that.
I have been keeping record of Finn and Fiona's heights as they have grown. I string tested Finn at 6 months initially, and Fiona at about 3 months initially (and have several times since with both - the numbers have remained constant.) Fiona has been consistently 3 inches taller than Finn was at the same age, throughout all the measurements since her birth - but Freyja was also malnourished when I got her, and was likely malnourished throughout her pregnancy with Finn (he was also subsequently weaned at 4 months) - an obstacle she didn't face while carrying and raising Fiona (and Fiona wasn't weaned completely until nearly 8 months).
I do have a measuring stick - complete with the little level built in - I picked up pretty cheaply. If you go to tack auctions, you can usually find them for about 25 bucks.