See SA OC 181, 9 in Baer Heytev in name of Shilte Giborim:
וגם כח אדם בעינן כמו הראשונים. מים אחרונים אין צריכין שיעור
... There is no minimal quantity for mayim acharonim.
But the function of the mayim acharonim is to clean the hands, at least two phalanges in each fingers (sayf 4). So a bit of common sense is needed to check if the process is a process of cleaning.
The Seder Hayom (¹) writes that a small amount of water makes the hands more dirty than clean. And if someone thinks to make a chumra when taking a symbolic mayim acharonim he is wrong. Additionally according to the last Sayf of the siman if someone will wash his hands after the meal the symbolic mayim acharonim are not valuable. Mishna Berura sk 10 says the same thing.
Note, when I was writing the answer, the answer of Danny was not already published. The answers are similar but the conclusions differ
(¹)
ואף על גב שלא נתנו רבותנו ז"ל שיעור למים אחרונים, אין ראוי לתת מעט מים על ידו אחיזת עיניים ולשפשף זו בזו כי אדרבה מלכלך אותם יותר...