Wiring a sub-panel in my garage 100' away

I am going to run power to my detached garage that is 88' from my house, but I keep seeing conflicting opinions on what size wire to run for 100A service. It looks like I will need #2 THHN with a #6 ground, but can I go smaller, like #4? I'm only planning on using a 60A breaker at first, but since I only plan on running the wire once as it will be buried, I want the ability to add to the sub-panel as I will eventually be running power to our barn as well, which will only be a few lights and 2-3 outlets.



Concerning the conduit, will 1" be big enough, and is 18" burial depth deep enough?



Thanks!

Wiring a sub-panel in my garage 100' away