Baileyworks here as well.
I have a medium pro bag and it is huge IMHO. Very well made.
I frequently use multi mode commutes that involve loading my bike onto a bus or into the back of my van. I keep things off of my bike to make this easier.
Did I forget to mention...my baileyworks is one comfortable bag! I use mine on centuries and out of the 200 miles I ride each week it is usually with me for around 150 or more.
I think for what the OP is pondering hauling a large would do.