The most suitable capacity of a small backpack is 30 liters, which is large enough to put down essential items and extras for personal climbing.
Small backpacks can be as small as 10 litres or as large as 40 to 50 litres.
If you often lead the team? Or help teammates carry things? Then you may need a backpack with a capacity of 40 liters or more.
In most cases, a backpack capacity of 30 liters is a standard capacity and the most popular choice.
But if you have activities ranging from hiking in summer to skiing in winter, you may need more than one small backpack.
Consider your needs to decide your choice of your little backpack.