The size depends on the design you choose. In general I choose the smallest hoop fits the designs. Especially for kids clothes you may need a medium or a small one, because there isn't so much space to hoop the fabric.
Sometimes if I can't hoop it I fix the fabric only with stitches or pins on the hooped stabilizer, good is to fasten with some tapes like wondertape or anything like adhesive tape. Is it a precious fabric I prefare to stich by hand the fabric on the backing - it's more safe and I can fasten everything around the stitching area.
Happy stitching and enjoy surfing around at cuties!
There is a lot to find, to learn and to share. Giving and taking - that's great here
Blessings
Bärbel

cherrysue, Bärbel has given you some great advice - use the hoop cloest to the design and on small clothes you have to use a smaller hoop - because of the size of the item you are embroidering on. Hope you share your work in projects with us.

Well I am just jealous that you even have an option, lol! My machine only uses 4 by 4 and I'd love to do larger designs! Happy embroiderying!
Debra

Cherry choose the design that will fit your hoop. Mostly I use a small or med. hoop for children. Most times I machine baste the fabric to the stabilizer & hoop only the stabilizer first. This works best for me. *