When I first hit the road I started with the incredible North Face Terra 45L. It's an amazing bag with all the right straps and none of the useless ones, sturdily built, and unfortunately discontinued. 45L fit all my summer gear and worked alright, but as I got into my groove I found myself struggling to fit a couple days groceries in. For that first day after getting food I was always bursting at the seams, or worse carrying a tote bag of food! I upgraded to Osprey's Exos 58L and I'm as comfy as I could want with food space. Even when I'm fully loaded the bag isn't full, which gives me hope for trying to pack for winter (false hope)!
I don't think I'd go to 65L as an everyday bag. I could see myself winning the lottery and using something that big or bigger on an arctic trek or something, but thats about it. If you do go that big you probably don't want to fill it all the way, just because it'll be too heavy! Big bags also make hopping trains and fitting into tiny cars harder! If you pull the trigger just keep in mind that having the space doesn't mean you have to fill it.