You guys are making a strong case for hydro, 
. Personally I don’t think my life is ready for that jump, personally, spiritually, or financially, lol. But strong case, none the less.
But for peat, my brain tells me 5g outside, 3g inside, 2g for a packed house, 1g for seed making?
I would say 5 gallon max for soil/dirt and indoor and autos. Outside - if room isn’t an issue give ‘em what you got. The best grow is the one that’s inside your property line 
Go with what works for YOU and your situation your plants AND you will be happier for it.
Okay so I decided on 3 gallons. But I’ve got some 7 gallons I may put 5 in for comparison. Small fabric bags dry out too fast around the edges for me. I feel stupid pricing up plastic pots then realizing the plastic bags are cheap: https://www.amazon.com/gp/aw/d/B08XB95621
Seeds should start coming up today, hopefully they haven’t started flowering already.