Welcome from Michigan.
The pin size does not really matter. However, larger pins will be easier to get through all 3 layers (top, batting, back) of the quilt. If you plan to pin at 6" intervals, you'll need around 140-150 pins. More pins, if you place them closer together. I calculated 10 pins per row with 14 or 15 rows for a twin sized quilt. I haven't pinned in a while, so my calculations could be a little off. I have a frame and short-arm setup now, so I don't need to baste.
Another option used by some of the members here on the board is basting spray. I've never used it, so I can't comment on it. But others do use it successfully.