How Big A Backpack Do I Need For A 3-5 Day Hike?

Pile up all the stuff you think you want to bring for you 3-5 day hike.  The pile will be enormous, so whittle it down to the stuff you think you can't live without, and figure out which stuff will go in your pack and what will be in the outside pockets, or tied to the outside of the pack.

If nobody else is really going to see your underwear for 5 days, do you really need 5 extra pairs?  Can your cooking pot double as a serving bowl? Can anything be safely packed inside something else?

Try to pile up the stuff that goes in the backpack in a reasonably square-ish shape.  Get a tape measurer and measure in cm the 3 dimensions -- height, width and depth.   Multiply these three dimensions together to get cubic cm,  Divide the answer by 1000 to get litres: most backpacks are sold with their capacity in litres indicated.

Example:
Pile size = 25 cm x 50 cm x 40 cm = 50000 cubic cm.

Divide by 1000 = 50 litres.  That's the capacity you'd need.