In reply to Sans-Plan:
> Osprey Transporter Zip Top is what i use, can get decent amount of kit in it.
> But in my experience as long as you don't have a 50L pack towering over you and stuffed to the max on your back nobody will blink an eye at it.
In my experience nobody will blink an eye at anything you bring as a carry on, regardless of a) which size you booked or b) whether it's in the limits. Ideally spend 10 mins walking up and down the aisle complaining there is no space for it while delaying the plane for maximum effect.
To the OP: if it's the baggage included in the ticket, so long as it fits under a plane seat it's fine. I have a soft 25L generic Lowe Alpine sack that works a treat, even with 3-4 days (warm weather) clothes, a mirrorless camera, and a laptop.