In reply to droites:
The zip is sealed/weather tight. I've not had a problem with it leaking but I've not yet put it through it's paces in Scotland. I still use a dry-sack to isolate dry gear from the wet.
Size wise, I'd consider it fairly large - I usually tighten the strops to prevent contents moving about. The hood attachment can be fitted in various positions from high to low, coupled with the drawstring at the top, it can be packed out quite a lot if necessary.
This is the only 45L I've owned so can't compare.
I can measure length & base dia. this evening if you want