Anything between 15 and 30 litres should be just fine if you live alone or with one other person and don’t throw away an above-average amount of stuff.
A 30 to 40-litre bin is generally advisable for families of three whose bin gets taken out quite frequently.
The best bet for four people or more — or those who don’t want to take out the rubbish any more than they have to — are 40 to 50-litre bins.
Go for something that's more than 50 litres if it's an all-in-one unit for recycling and general waste.