First of all, you should probably post this in the heating subforum, not the main forum.
If you do that, some polite person will reply and tell you that without a heat loss analyis...your question is unanswerable.
Then...someone else (maybe me) will suggest that even if we did know the btu input of your boiler, we can't tell you if 1/2" is big enough without knowing the length of the pipe run to the garage and FOR SURE if you're talking about propane or natural gas. I'm assuming (perhaps wrongly) that if you even THINK 1/2" might be big enough, you're talking about propane...but that's a big guess.
Good Luck with your project!
Phil
