Some key factors when reading 225.39 -- is this detached garage just a garage or a dwelling with a garage (e.g. is there an apartment in it with living/eating/sleeping quarters)?
Next is there really a panel there? If you just have one circuit or a multiwire branch circuit, things are much simpler and just a switch will do.
But if it is a detached garage, just a garage, with a panel, you'll most likely need a 60A disconnect (but it could be a 30A, depending on which part of 225.39 applies).
Final complication is does the panel have a single main, or are you using the 6 handle rule? The rating of the disconnect is the size of the main or the sum of the up to 6 disconnect breakers.
Finally, the size of the feeder at the main building is not required to be as large as the disconnect at the remote building (e.g. house could have a 30A double pole and garage could have a 60A double pole main). This last item seems to cause a lot of debates with inspectors, but 225.39 clearly says rating of disconnect and not rating of feeder.
