I've just brought my backpack for my RTW trip im doing early next year, the backpack is a 60+20l. I was wondering how strict budget airlines are on handluggage and would the backpack be able to be considered as handluggage to save on costs of checking the bag in?
That sounds a little big to me, but every airline is a little different when it comes to carry on. Just go to the airline's website. I use a 40L pack and I believe I'm near max for most airlines.
(suggest you Google "carry on measurements")
Budget airlines are quite strict, and that backpack definitely sounds like it's not going to meet the hand luggage measurements. Even my 35L bag barely fits according to the official sizes.
Ryanair measurements for example are 55x40x20 cm. If the bag is 60-65 cm, that still doesn't matter too much, but I guess in this case you'll have to pay and check in. KLM uses more or less the same, but gives a smaller maximum in thickness.