How much space in bags I need for 4 days trip

I want to go on bike for 4 day so i will pack very light, but I don't know very well how much space i need in my bag

I maybe seek for the Axiom Seymour Oceanweave P35+ or Ortlieb Sport-Roller Free, do you think I have enough space for a light trip? I will mostly bring clothe and some meal

I fear that the Axiom Seymour Oceanweave 22+ wil be too small?

Also I think about bring my laptop, any recommendation for a case?

I think add this rain cover to protect, do you think it will fit https://www.decathlon.ca/en/p/8734213/hiking-backpack-rain-cover-2040-l ?