The belt size it taken round the outside. Stock a50 belt length is 729mm long, 17.7mm wide and 30° pitch, so you would want a 729 -17.7-30 belt but a 729-18-30 will work pretty much the same.
I currently run a 736-17.5-30 belt on mine with my race variator and gets me another 4-5mph downhill on a good day. Some folk have run 743 belts on Malossi Multivars and i think i remember someone even using a 748 long belt. If the variator is stock the 729 belt will prob be wnough. I just like messing about, i just got 3 new belts yesterday of various sizes so i can try them out, ones a 729 stock belt icwanted just to keep as a spare.
The biggest problem is that many belts listed as 729 are NOT 729 long. Seems every manufacturer has their own standard. I have at least 2 belts i got in the past that didnt measure up to what was stamped on them. If you check the brand of the belt thats in it and even better get the atock number off it then you can get that exact belt to replace it. Or if you have a race variator fitted try a slightly longer one, the 736 works well for me, best size i found..........so far.