This backpack measures 55x40x20cm and weighs 680g. The size is the maximum for a Ryanair carry on bag.
Ryanair have a strict policy off one carry on bag per passenger (including bags, shopping etc). They also have a 10kg weight limit. If you have a trolley bag, then it can weigh 2-3kg on its own. They also charge £30 to check a bag into the hold for a return trip when you place the booking. If you have to put the bag in the hold when you get to the gate, then the charge is £35 for a single journey.