From experience, a filled 100-litre suitcase will be well over the 20-kilo allowance. My own suitcase filled with clothes, shoes and toiletries always ends up being between 17 to 19.5 Kilos.
It is a 66-litre suitcase. The dimensions are 67cm x 42cm x 24cm (extendable to 27cm if you unzip the top of the suitcase to give you more space).
Once I’ve packed my suitcase, I always put it sideways on my own weighing scales to ensure it is well below the 20Kg allowance. Otherwise you’ll find you have to pay a fortune at the airport for each extra kilogram. Of course, some airlines let you pay in advance at the time of booking for an extra 3Kg, if you know your suitcase is going to be more than the 20Kg allowance.