Originally Posted by Steve B.

Not sure what the magnetic clip is.

How much stuff do you want/need you carry beyond water ?, as in, some places you want a rain jacket as it gets cold at altitude. That drives the size. Had a really old CB Mule, could handle 100oz but no room for a jacket. I then found a deal on a Quest pack from Dicks, paid $20 for it, usually sells for $50. It’s actually a really nice pack, has comfortable shoulder straps and cross chest clip. I don’t like the bite valve on the Quest reservoirs so I just use my old 100oz CB reservoir, as I like the CB BiteValve. I actually hate the CB reservoirs whose bladder cap is some odd line-up-the-tabs system and that I’ve failed to properly close on 2-3 occasions, with a subsequently drained bladder. I prefer a simple design I can get ice cubes into and that screws shut. Generally I’d say get big as you don’t always have to carry stuff, but when you do it’s nice to have the capacity. The Quest fits that niche for me.

Edit: A normal "road bike" standard is about a 20oz bottle per hour. I tend to drink more on the mt. bike especially on hotter days, so a 25 mile mt. bike ride @10 mph = about 2.5 hrs of riding, so maybe 70 oz would be minimum ?,