It just depends on how big you want the bows. The measurement refers to the girth of the ribbon used.
The easiest way to visualize the size of the bows is to think in terms of dogs in shows.
3/8 bows are used on the Maltese double topknot. They are very small.
5/8 is the size that is usually used on Yorkie pups in the ring.
7/8 is the standard size of bow that is used on adult Yorkies in the ring.
IMO I like a 5/8 bow if it is just a fashion accessory.
I hope this helps a bit.