I have a 60 amp breaker panel in the middle of the house 25 feet from the meter. It currently has 8 gauge wire that runs into and through the attic, over to the panel and down into the panel for a total of about 36 ft of wire. I want to upgrade to a 100 amp main breaker panel at the existing location because I don't want to have to run the house wires back to near the meter. What's the minimum size wire I need for the hot, neutral and ground between the meter and the panel?
Depends how you feel about copper. Aluminum is the better choice for feeder, lighter and more flexible too, but you need larger wire section if space is a huge issue. Once aluminum wouldn't fit in the pipe and I had to use 1000 kcmil copper... you don't even wanna price that, let alone pay for it...
Anyway you use 1 AWG hot-neutral if aluminum, and 3 AWG hot-neutral if copper. Or larger but that's the minimum. For the ground, #8 copper or #6 aluminum.
Since I gather that 60A panel was super small, you can see where spaces are the most important trait of a panel. Spaces are dirt cheap when you're buyting the panel, but getting too-small a panel is an expensive and easily avoided blunder. Once you grok that, your thrifty side will root for the biggest panel in the store. Now, to clear up a common misconception:
Panel ratings are like tire ratings: They're absolute maximums. If you drive 85 mph everyday, you'd want a 112 mph tire - an 85 mph tire leaves zero margin of safety. Lots of people buy 100A panels because they think they must match the feed breaker - but they're just running the panel at redline. A 200A panel gives a margin of safety.
Main breaker rating in a subpanel Does Not Matter and does not need to agree with the feed breaker. It works fine either way. In fact, subpanels never need main breakers. (however subpanels in outbuildings need a disconnect switch, and a breaker covers that).