How much water must there be in the vessel used to pour? [1]
The vessel used to pour the water must contain at least a Revius[2] worth of water prior to beginning the first pouring. [If it does not contain a Revius worth of water, the washing is invalid.[3]]
[1] Siddur Seder Netilah; Kama 4:1; 159:3; 160:16; and Seder Netila 12 regarding washing for bread.
[2] The Grach Naah writes that it is 86 Milliliters or 2.9 fluid ounces. This is the same Gematria as the word Kos. [Shiureiy Torah 3:6]; Others say that it is 105 Milliliters [3.5 fluid ounces]. Others say it is 109 milliliters or 3.7 fluid ounces. Others say it is 115 milliliters. Others say that it is 128 mil or 4.3 fluid ounces. [See Koveitz Yagdil Torah NY 60 p. 263-264 in a Ksav Yad of Tzemach Tzedek in name of Admur; See Kitzur Hilchos Shabbos Miluim p. 18] Practically the widespread custom is like the ruling of the Grach Naah.
[3] Based on Siddur Seder Netilah; and so rules: Biur Hagra 4:7 in name of Rieh; Rashash in Nehar Shalom; Ruach Chaim 5:5; Kaf Hachaim 4:26
Background of ruling of Siddur:
Admur writes in the Siddur that all the laws of washing for a meal equally apply for the morning washing. In 159:3 and 160:16 Admur rules regarding washing for bread that the vessel must contain a Revius of water for it to be valid and hence the same would apply for the morning washing.
Ruling of Kama: In Kama 4:3 Admur rules: “If one does not have a Revius of water available he may nevertheless wash with a blessing being that the Sages did not differentiate [in the amount of water], as one is nevertheless washing with valid water.” This follows the ruling of Michaber 4:6