[Ibby]

I'm assuming that "bridge pin" implies steel-string acoustic?
When you say it's very loose - does it hold tune? is it too loose when brought to tune, or does it not get up to tune?
If it holds tune but is too loose, you want a thinner string by a tiny bit.
If it won't get up to tune, you more likely have a problem with your bridge rather than your string. Make sure the bridge pin is being replaced properly. Honestly, the string should break before the bridge gives out, if the string and bridge are installed properly and its a quality axe. If it won't tune, and the bridge pin is definitely going back in right, you can try replacing the string but I doubt that's the problem - more likely you'll need to replace the bridge pin or even the bridge.