There is no set belt size. Every car is going to be different depending on which brand UD's you have, if you used it on the alt pulley or left the stocker and other factors.
Best thing to do is get a string, wrap it around the pullies and measure it. Then buy 3 belts....one the lenght of the string and one above and below it.
WHen you put the belt on, you want it to sit on the tight range of the belt tensioners scale, because the belt will stretch slightly.
Then just return the belts.
Sorry i couldn't give you a solid number, because every car is going to be different when running aftermarket parts.
HOWEVER, if you are sure you have ALL 3 UD pulleys on, and are running the smog pump, just have them look up a belt for an 87-93 WITHOUT A/C. It's a good starting point because more UD pulley sets when using all 3 pulleys keep the original belt. However, if you don't have the smog pump or want to bypass it, or only have 2 of the 3 UD pulleys, you'll have to string it.
What are the numbers on the belt?? Usualy the last 3 numbers are the belt size in inches, so if it's GDYR800845, it's usually an 84.5" belt.
SO if it's short an inch or so, you cna usually go in and ask for say a GDYT800865 belt which would be 86.5 inches
If the perseon behind the counter is on their game, they will understand what you are trying to do. Just make sure you tell them you have custom pullies and deleted accessories so they know it's not a stock setup.
One time i tried to get a belt and the person didn't understand the numbering and kept referring me back to the computer so he could just give me the OEM belt. They couldn't grasp that i no longer had factory accessories or pulleys