For the road, don't bother. You can fit all you need in your jersey pockets/saddle pack/frame mounted bottles. The smaller camelbacks/generic type packs that would suit, ie two litre - do not have the carry space. If you can get one with larger pockets then weight on your back, heat (sweatiness etc) becomes an issue.
The other tip I've learnt is look at the weather forecasts before you ride (not just the BBC!!!) Why carry a large coat/waterproof when its going to be fine, when a small gilet and arm warmers may do - and they can fold away tightly to fit in your jersey pockets.
I have a
Leisure lakes branded hydropak for a bargain £15 quid some time ago and this takes a two litre bladder and has a pocket for some spares, but any spare clothing has to be straped under the saddle. The heat and chaffing/weight made it unbearable for the road, but mountain biking with frequent stops is ok.