1)
How do we explain "Mayim bi'Mesurah Tishteh Shishis ha'Hin"?
1.
Malbim (from Bava Metzi'a 61b): Mesurah is a small measure, about one part in 36 of a Log 1 . It is not a sixth of a Hin (two Lugim). Rather, each time you will drink only Mesurah, and not be satiated, and within 24 hours you will drink a total of a sixth of a Hin. This is a sign for Bnei Yisrael - so they will do during the entire siege.
1
This is about 9-16 milliliters. (PF)
2)
What do we learn from "me'Es v'Ad Es Tishteh"?
1.
Rashi: You will drink this amount every day.
