I just sized up from a 36 L to a 50 L to accommodate a growing trad rack (still small-medium sized, with doubles of most). I found with my 36, though ideal for sport climbing, was never quite enough to run a full rack. If packed JUST RIGHT I could fit all my gear, rope, harness and 1 pair of shoes; leaving no room for extras: helmet, additional clothing, 2nd pair of shoes, or even food & water.

Hope this helps, but I don't think I would suggest anything smaller than a 40 L.

Black Diamond is generally awesome. I use the BD Hollow Point for simple sport climbing days where I carry my rope in a separate bag--and the wall is not far from my jeep. The bag I use for longer treks to the wall with more gear, such as a trad rack, is the Arc'teryx Miura.

For comparison sake, the Miura is fine and easily will hold your gear but lacks an important feature BD typically integrates into their packs:

--Lots of separate small storage pockets (sometimes you need to be able to organize your knife, lip balm, headlamp, gum, camera, keys, and other small essentials)

Be sure to look at the fine details of the packs...those extra bells and whistles make organization much easier, which makes a difference in your experience outside. Peace