Subpanel rating >= feeder wire rating >= supply circuit breaker
Tires come in several speed ratings. 85 MPH, 112 MPH and 130 MPH. Subpanels come in 70A, 100A, 125A and 225A. It's only a "never exceed" red-line. You don't have to drive 130 MPH or 100A :)
I for one am not a fan; the truly valuable thing in a panel is breaker spaces, and we recommend plenty of them. You're more likely to max out spaces than amps in most cases. 100A panels are notorious for not having very many breaker spaces.
On subpanel feeder, we like spending safety money wisely. Since the lugs at both ends are aluminum, and since large aluminum wire is proven safe, we like stuff like 2-2-2-4 aluminum, which is an inexpensive commodity and 90A.
Since your distance is only 80 feet, voltage drop won't be a concern and there is no need to upsize wires for that.
One of my favorite money-savers is 100-200A panel (for the spaces) fed by 90A 2-2-2-4 (for the cheapness) fed by a 60A breaker (for the cheapness). The 60A breaker can be upped to 90A at any time. This will cover almost anything folks do in a garage, especially, charging EVs. Most people new to EVs learn a ton of stuff that's dead wrong, but we can put you right and save you a ton of money vs that wrong stuff. And also not burn your house down :) So if EVs are in your plan, ask us for advice/review before you buy any wires. I'm talking to you, passive reader!
No need for junction boxes
The 4-wire feeder from house to garage does not need any intermediate boxes or splices - except - that to the extent it's routed in conduit, and single wires must be routed in conduit, the conduit needs access points enroute to facilitate pulling. With rare exception, conduit must be built empty and the wires pulled in only after the conduit is complete, anchored and tamped. Access points must remain accessible forever. They can be above drop ceilings but cannot be plastered or built over.
4 wires for real. + ground rods.
A service is not the same thing as a feeder.
The electric service from your utility is 3-wire because the utility has no business supplying ground. You need to originate that locally, and make ONE bond between neutral and ground in your entire system.
The feeder to the garage needs to be 4-wire. It needs to be able to return enough fault current on ground to trip a breaker, and dirt can't do that. (that's why we bother to mine copper and aluminum.) Neutral and ground can't be combined - we tried it for many decades, but the reality is, when you combine them you don't have ground anymore - you have neutral. And attaching metal chassis of things to neutral is a recipe for fatality.
The ground rod is also required. They do different jobs. The rod is more about dissipating ESD and lightning, and assuring that the dirt near the outbuilding is near the same voltage as electrical grounds.
