From a bike touring perspective, I use 1 liter water bottles. The ones I use are the disposable ones sold in USA under the Smartwater brand or the Life WTR brand, but I do not dispose of them, I re-use them. The 1 liter sizes fit nicely in bottle cages, but the smaller sizes do not. They do not come with convenient lids, I use the flip top caps that I obtained off of other sized water bottles.
The bottle under the downtube, I strap something around the top to make sure it stays in the cage, the bottle is heavy and the center of gravity is pretty far forward so there could be risk of bottle coming out of the cage. But most rando bikes would not have room for a full 1 liter bottle under the downtube, so that is less of an issue.
The disadvantages of them is the tall size makes them too tall for smaller frames, the small opening on top makes them slower to fill, and they are too tall to be able to fill them in some smaller sinks. But when bike touring, I am not in as much of a hurry. If you also had a small bottle, and had to fill the bottles from a small sink, you can transfer from the small bottle to the tall ones.
I did a bike trip in West Texas where it gets hot, I carried a couple of the one liter bottles in my saddlebag besides the bottles on the frame.
