When browsing the toys, look to the left side of the shopping pages and you'll see a bunch of filtering options to help your search, including things like length and girth. For example, here
is a selection of dildos that have a usable length of 6" or less.
Typically the G-spot is between 3" and 5" into the vagina, and occasionally it'll be shallower or deeper. It's different for every woman. If you already know where yours is, then that's awesome. If you need a toy to help, my personal recommendations are the Ella
or the Bent Graduate
. Both are of an un-intimidating girth and made of body-safe materials, and have a great curve which can help quite a lot.