Another thing you could do is
(again, while the correct procedure is followed - ball end into hole, feel the edge of the ball end softly touching the edge of the bridge plate, keep the string more or less still while you insert the pin so that the latter's tip goes past the ball, pull the string tight while you thrust the pin in completely)
turn the pin so that the slot is *not* at the front (old-style pins didn't have slots at all), install the string as above, shove the pin tight (it will inevitably sit higher than before, but it should be stuck snugly, even though the pin head will still be loose at the top).
The above having been said, bridge pins don't break banks, and there's something to be said for having everything fit as Herr Martin intended.
And no, I do not believe there is a standard pin size for acoustic bass guitars. Trial and error (or luck at first try).