I got my first white belt with my first gi. The belt was a good fit (I guess) and I did not pay much attention to the length of it. The gi was Tatami (size A2) so probably the belt was also A2.
See also: BJJ Gi Size – how to get it right.
I got another white belt with Ippon gi, A2 again. As our BTT gis come without belts then I left it on the hanger in the gym so that someone else could use it. And, it found its new owner quickly. The third one I got with Progress Academy gi, A1L this time.