Okay, so the different sizes confuse me :). I have a couple 45 mm ones, but what are the 60 and 28 mm sizes used for? Does the size of the blade affect how much area they cover? (if that makes sense.)

I think the 28 mm ones are for 'tight' spots. I have not tried the 60 mm ones as I don't see the blades in stock very often so I just use the 45 mm one which seems to kind of be the commodity item one - most readily available replacement blades.

I have all three of those sizes, I use the small one for garments (small curves, find the 45 worthless, and love the 60 for rotary cutting quilts.

Compare the size of the blades to sizes of tires -

the 28 mm to a kid's tricycle wheel/tire
the 45 mm to a car wheel/tire
the 60 mm to a semi-truck wheel/tire

The very bottom line is that they all cut fabric one thread at a time.

The bigger ones (60 mm) are better for thicker fabrics and more layers (think trying to get through snow or mud)

The smaller ones (28 mm) are better for tighter curves than the larger ones. (think of a shovel instead of a bulldozer)

For most people, if they can only have one, the 45 mm is the one to get.

I have the 28mm I use as a seam ripper. It's the perfect size. I found a teen tiny one at Walmart and it's good to use in the slotted rulers. I use the 45mm for most of the cutting I do.

Thanks for the responses, everyone! I'm thinking a 28 mm might be of interest to me because I've been doing a lot of detail cutting lately. I really appreciate it!

My main cutter is the 45mm. I have cut 18 layers of fabric and not had a problem. The blades are the easiest to find.
Sue

I have all 3 sizes and use them for different things. Mostly I use the 60mm. If you watch eBay you can find good deals on the cutters and blades. Sometimes you can find multipacks very cheaply. I got the Olfa 60mm and a dozen or so blades for under $30.

No matter what size the fabric or block size is, my main rotary cutter is the biggest. That's just me and I feel comfortable using this size.

The 45 used to be my favorite size for cutting, but not anymore.

the large is good for flannels- minkey- any thicker-heavier fabric-
the smaller one is good for curves, circles, small applique shapes you are using templates for.
there's one smaller than the 28- i think it's 16- it is the one on my circle cutter.
they each have their place- the basic for quilting cottons is the 45- just a nice size to work with- that can handle the bulk of the jobs they are needed for