Kellie, how's grooming going? Hope you like.
4FC leaves 3/4" of hair. I like this best for Yorkies, not too long, not too short for the summer. You can also go w/ 5FC which leaves 1/2" on the body. I prefer the 5FC for my cottony boy.
I like using 7FC for cats. It leaves 1/4" of hair, but in reverse it will leave half of that, which is 1/8", what a 10 blade will leave w/the grain. I find 10 against the grain to be too short. Hope that helps.
F stands for finish.
PetEdge: Andis UltraEdge Clipper Blade Size 7FC
Ceramic will last longer & stay cooler longer, but will need to be sharpened more often than the UltraEdge. Since, it's for home use, go w/UltraEdge blades.
You don't want to get the skip tooth ones, they're more for blending & if not used correctly, it can catch the skin & cause serious damage.