It's a different seatbag in the pictures – the first one is the Highline, which is a bit smaller and designed to work with full-suspension bikes. The second one is a 7" Picketpost (it also comes in a 9" version), so it fits hardtails with at least 7" of exposed seatpost.
The Picketpost is super stable – I think it's the most stable seatbag on the market, from riding with friends who had other seatbags and comparing. But of course it only works if you have enough exposed seatpost and you're riding a hardtail. The Highline will definitely sway more, but I have a couple that have been field-tested, and we haven't had any leg-rubbage yet.
I haven't had time to do a good volume measurement, but when I get the first set of bags in April I'll do a proper scientific test. Probably by filling them with water and stepping on a scale. I would guess 12 liters is a reasonable amount to put in one – you could probably stuff the larger Picketpost with 14-15 liters.
The bag is about 6" wide and 6" tall in the rear portion, and skinnier near the seatpost.
You said you're interested in handlebar storage too, so you should know that I'll be doing this same thing with my handlebar rolls in a week or two – they have the added advantage of being seam sealed for full waterproofness (well, as far as I can tell – the washing machine test is forthcoming). I'll post it on facebook and BPL again, as well.
