I use a 9 in, 1/2 nap for almost all that I do.
It depends on about a million different things to answer you're question.

The smaller rollers hold less paint than larger ones, so you have to return to the paint bucket or pan more often. The larger the room, the longer it will take using the smaller roller. IMO, a small bathroom, a closet, or a single wall are the limits to the skinny rollers. I don't like them much because I try to maximize the paint on the roller and end up with drips.

As a small woman, when I use a full-size roller, I use a 6 inch roller. The reduced weight and size give me more control.

I rarely use small rollers. Only when the area is small enough that a regular roller would not be able to properly spread the paint. I do find they work well for paneled doors to apply paint easily, then use a brush to smooth and cover details.

The guy that's painting the kitchen has a 15 inch roller. I might try that next time I need to prime/paint a whole room, he loves it. Also seems easier to tape switches and outlets and just roll over them. Will have to try that too.

In my opinion you use full size roller.

Only when a larger roller won't fit easily. If it's a small space like a closet with shelves, I might use a mini-roller for the whole thing, but I like larger rollers because they cover faster.

Last time I used a 7" roller was priming insides of base cabinets. 9" was too clumsy, would not fit between shelves, wiz roller held too little primer. Did use a wiz for the (oil) finish coats there, though.
Casety