i got mine from argos its was a 15 litre one does the job for a 2m by 3.5m room that has the res and veg tent in. i drain in to my res because i use auto pots
To determine how big a dehu you will require, figure out how much water per day the plants take during peak drink, not including runoff unless it's left standing in the space (no bueno). 97 - 98% of that water will need to be dealt with by the dehumidifier. I can't recommend any specific products as I am in the USA.