Most manufacturers mention the capacity of a tote on wheels in liters. But that’s a tricky way to gauge its capacity and you might end up buying a smaller one than what you’d actually need.
For example, 20 liters sounds like a lot. But in reality, it’s a very low capacity bag. Instead, check the maximum weight capacity of the tote. If it’s rated to carry 40 kilograms of luggage, you get a clear idea of the amount of luggage you can stuff in it.
It’s a given that your tote will be forced to gulp more than it can swallow. So, always plan for extra room for the things that you might carry irrespective of whether you need it or not. The caveat is that higher capacity totes are not airline travel compatible. If your travel plans include stowing the tote in overhead bins or under the seat, then ensure that they comply with rules set by the carrier you frequently patronize.