How small is too small for RO?

[maple flats]

Can I use a RO system that is made for under the sink tap water? If not, what is the difference?

The big difference is that they are set up opposite and are likely too small even for your size operation.
The under sink units throw out the concentrate and keep the permeate, the opposite of what we need and most are real small, rated in gallons per day (24 hrs non stop).
While one could help a little, you will be far better off buying or making one for maple.

[30AcreWoods]

How much sap do you need to process? My little RO would easily push the concentrate 30-ft...but you'd need to store it if you are running overnight. One simple solution I've used is a 55-gal food grade barrel on a chunk of wood. I cut a valve in the bottom and use it to hold concentrate overnight. Word of caution: boil immediately as the high sugar content of concentrate means bacteria grow super-fast, so the sap will spoil quickly.

[DRoseum]

RO is great time saver. If you go to a 3 membrane set up, make sure you size your pump accordingly to get enough flow at high pressure (~100 psi on most 400 gpd membranes). Also I would highly recommend setting them up as a 2 stage set up. 2 in parallel and the third in series, combining the output of the 2 in parallel as its input. RO_Membrane_One_Two_Stage.jpg