[Closed] Weekend rucksack (as opposed to day sack)

I'm off to Wales for the weekend shortly. Travelling by train, so I'll need to be able to cycle to Paddington with a weekend size bag on my back.

What's a sensible size (in litres) for that would you say, given that I need a) to be able to manage the weight and b) be able to see behind me to check for traffic?

I'll pack a Camelpak for riding with when I get there, plus a few pairs of shorts, jerseys, basic tools and spares and a credit card.

Any specific recommendations? Seen a Berghaus that looked about the right size but didn't catch what it was called and fear they may be spenny...