A larger dome can still come off the aid, but is less likely to come off lodged midway down your ear canal, because the dome’s widest point is wider than your ear canal. If a larger dome does come off the aid in your ear, it will be closer to the opening of the canal, so it wouldn’t be sitting as deep in your ear canal, and you can probably pull it out with your fingers.
The sound may be too occluded for you with a larger dome, though, or you may not like the feeling of fullness, or the audiologist may say, no, this is too big for your ear. But it’s a quick thing to try in the audiologist’s office.
And again, if the dome is matched and installed correctly on a newer aid, and you check to be sure it’s really on, it should stay put on the aid when you gently pull the aid off your ear. (Don’t yank it out.)