It'll use either the G-17 or G-19 mags, if it's a 9mm, or the G-22 or G-23, if .40...
If it's for the G-19 or G-23 mags, the larger ones will still work fine.
Most likely you'll use the full-sized mags (G-17 and G-22).
Measuring the rear, right corner of 2 post ban, 9mm, 10 rounders I get:
G17 - 4.75"
G19 - 4.12"
If they are factory Glock post bans, they should be clearly marked at the top of the back of the mag with either 9mm or .40 CAL. Some early prebans weren't marked. If your SUB takes the mid sized mags, the full sized mags will also work but will protrude an extra 1/2" out of the grip. ie. G17 mags will fit in a G19 grip... won't work the other way around.