It is very small, which is part of the reason I bought one. I still managed to get the essentials in; pump, spare tube, tyre levers, puncture kit, multi tool, couple of spare chain links, led flashers, windproof jacket, spare base layer/storage for a layer if I get hot, couple of gels, couple of chocolate bars (boost/starbar), bag of dried fruit, bag of jelly babies, bank card, £10 note, mobile phone, keys.
I seem to drink a lot when I’m out on the bike and 2L was never enough. I always took another 750ml bottle with me anyway.