Electrical – Construction heater in garage

It can only be 3 if this is the service drop - if it's a sub-panel it needs to be 4-wire.

3/0 (000) should do it. There are MANY voltage drop calculators on the web, as well as tables/charts. I happen to like the one linked, but they are mostly similar and should all give similar results. Shoot for 3% or less drop at rated (or calculated) load. I added 10 feet to account for getting the wire in and out of the ground, rather than just the horizontal distance - wire length, not "as a bird flies" is the number that matters for length.

I prefer to allow for the full load (100 amps) rather than attempting to finesse 30A 240V dryer + 120 volt loads unknown; I've lived with an undersized feed and I'm not fond of self-induced brown-outs.

No need to worry, a 200 amp supply box has more than enough reserve to power your garage. If there ever was an exess draw - anywhere in the system - your circuit breakers would react by tripping.

You could run a sub panel to the garage, in case you ever wanted to use more powerful tools in the garage.

Install a 50 amp duael breaker in the panel, run #6 three-wire line to the garage in a plastic conduit line and connect it to a sub panel circuit box in the garage. Then you can run individual 20 amp circuits from the sub panel to power outlets in the walls and another circiut to your workbench, etc. you could run 3 or 4 20 amp circuits in the garage. This would leave you in good stead for any future needs in the garage.