In reply to Northernladlovesgravy:
Yes. Even if you are carrying rack and rope(s) and your partner is carrying only helmet, shoes and harness.
I used a 15 litre pack (with rope and helmet outside! And a third of a rack inside) on a 9 pitch trad route, with partner carrying the rest of the rack.
Just get the 45l or the 45+10. If something doesn't fit, then strap it on the outside.
Having said all that, there is something to be said for having an oversized pack so you can just chuck stuff in at the end of the day (or the start of the day with a hangover) without having to be neat and organised. Steve McClure subscribes to this, and I sometimes do too (I have an insane number of rucksacks of varying sizes and often favour an ancient clumpy 70l Karrimor thing I got for a fiver in a charity shop, over my snazzy Lowe Alpine Crag Attack 45+10)