One of the considerations while purchasing a backpack is the capacity of the backpack, the volume of luggage which can be carried.
Usually for online purchases the seller will specify the capacity of the backpack.
The standard size of backpack will have a capacity of 35 litres, 38 litres or 40 litres.
The backpacks are usually having 3 compartments and compartment dimensions will vary depending on the design
Usually larger backpacks are more expensive if the material and quality is the same.