RO units usually come in standard sizes raited by US Gallons per Day, for example 50, 75 or 125GPD.

 

If there are no labels on the outside of the unit indicating this then you can usually work out the size of the unit from the flow restrictor on the waste water line. Most flow restrictors have a number printed on them, this refers to the flow rate of the restrictor in ml/min.

 

50 Gallon per day RO unit  = 300 ml/min flow restrictor
75 Gallon per day RO unit = 400 ml/min flow restrictor
125 Gallon per day RO unit = 800 ml/min flow restrictor

 

If the flow restrictor is blank and you are due to change the membrane on the unit a label will be found on the membrane itself. Amongst the code on the label there will be the number to indicate its rating. It is important to change the flow restrictor and membrane at the same time, especially in hard water areas.