Because of swelling some of ours looked bigger after a neuter than they did before. About a week of swelling but up to 6 months before they shrivelled up to nothing. Varies. I'd only Vet if swelling continues longer than a week or if he seems to be bothered and in pain. Or if swelling is really pronounced. Ours all seemed to not notice it at all.
LOL, cats used to be neutered by castration that completely removed the testicle and the scrotum. The first time we had one done that left the scrotum I was so mad at the Vet, thinking he hadn't done the neuter so what the heck had he done?