Meaco Arete Dehumidifier 20 or 25?

IMHO it is better to have bigger - it will work less hard - when the desired humidity level is reached it will turn off. If you can afford it, go for the larger one. (edited)

We have a 4 bed and 2 20lt dehumidifiers (Upstairs and downstairs) that we usually have to empty every day or every other day. So definitely worth getting the 25.

Recently purchased 20l Meaco arête for my 3 bedroom. It’s bigger than I thought so just keep it upstairs in the landing. 
I was given advice here about opening windows bathroom and bedroom daily if possible for 20-30mins- that seemed to help with the condensation as well. 

Just to be clear the 20 and 25 litres relates to how much water they can extract in 24 hours. The actual tank is only 2 litres. I have a 4 bed end of terrace and had terrible condensation in and around my bay window. Even in the early days where it was running 24 hours a day the most I emptied it was twice a day. As such the bigger capacity just means you'd empty ever so slightly less often. I'd say the 20 lite should be adequate.

Do you know what's causing the damp? Is it just that the hall is too cold and the bathroom doesn't have good ventilation/heating? Is there an effective damp proof course given the age of the property?