How to chose the correct hydraulic disc brake rotor size for touring bike?
There's no one right answer.

Most get by with 160 front and rear. There can be some virtue in going to 180, especially in front, to help manage heat and ensure a good power level when descending mountains heavily loaded, if that's going to be a thing you do much. Not every frame/fork is strong enough or has clearance for 180, however. The forks on more MTB-ish/adventure/bikepacking type bikes will often be able to handle it, while more road type forks may chatter and flex too much.

Running 140 either front or back, as often see on road or cross bikes, would generally be not adequate for loaded use.