|November 21st, 2023
In general 27,000 mAh is a good upper size: past that and your airline might have limits (though the FAA allows airlines to permit up to 44,000 mAh). And once you're getting that big it probably makes more sense to get two for the flexibility and redundancy benefits.
I first got one in 2017, and it's still working well (though the USB-C port has gotten slightly loose):
I've since gotten a couple more so the kids can have separate ones for their tablets on long trips.
 The math here is terrible: capacity should be measured in watt-hours, but instead everyone gives it in milliamp hours relative to some implied voltage. For USB power banks that's (always?) 3.6V, so 26,800 mAh is 96 Wh. My phone is 4,385 mAh at 3.85V, so 17 Wh. My laptop is helpfully spec'd in Wh: 99.6 Wh. Note that you do lose some capacity to conversion losses, so discount a bit.