Since your trailer is more than 20 years old and you haven't mentioned if its undergone axle
replacement, I'd measure from the back bumper to the ground and let that measurement be your guide for purchase. Even if you get them too short... you can always add more lego blocks or wood under the foot pad. Personally, I took the Scamp
stabilizers off my 16 footer. Everytime I went to use them my hands got really, really dirty from the road rash on the stabilizers. And more importantly, the holes for the pins always seemed to be 1/2 pin too tall or too short and I was looking for perfectly stable/level. I gave them away and haven't regretted it one moment. In my mind there are better stabilizers available, but your mileage may vary.