In a gen1 ES 300 it holds 1.25 gallons.
I use rain-x glass cleaner (got a super concentrated 64oz bottle years ago) But it's ghetto stuff for real.
Made by Pennzoil-Quaker State
Chemical CAS No / Unique ID Percent
Surfactant(s) (unspecified) 000000-00-4 <10
Isopropyl alcohol 000067-63-0 <5
2-Butoxyethanol 000111-76-2 1-5
Water 007732-18-5 70-95
Which means that it uses rubbing alchohol to clean the glass, and the butoxyethanol leaves a "film" behind in an attmept to make it a perfectly flat surface. So it's nothing special.
Which also explains why Rain-X wills treak glass, if the glass is not cleaned enough.
You would do just as well mixing 1 gallon of tap water, with a $2 pint of rubbing alchohol. It's surely be a hell of a lot cheaper!