A moderator that is exactly made for your caliber will be its most efficient and quiet.
That being said it is still standard practice to upsize 1 caliber, this prevent pellet clipping and a potential turbulence issue, but only low end ones will have a turbulence issue like the cheaper 3d printed ones. Going on size larger also gives you the ability to use it on that larger size in the future and have 1 moderator for 2 different calibers.
My 22 caliber Donny Fl moderator is much quieter then the 30cal Donny Fl moderator on a 22 cal gun, the 30 cal moderator doesn't even really make a difference in sound level on a 22cal, it just makes it sound lower to the shooter, but the DB meter says they are the same noise level, its a audio illusion.
On My Hatsan 25cal QE since I left all the factory baffles in it I used a 30cal moderator to prevent clipping, If I would of eliminated the factory QE baffles then I would of went with a 25cal moderator.
I think ideally its a moderator for your exact caliber, but its just so much more economical and more handy to have 1 moderator for 2 calibers. I would only ever upsize 1 caliber,
a .22 for a .177, or a .25 for a .22, or a 30 for a 25cal etc. I wouldn't size up more then 1 as it just seems to be more looks then function at that point.