The exact size of a seidel depends upon the place and time. The November 1890 issue of the St. Louis Medical and Surgical Journal says it equals a quart. This 1878 table of equivalencies more precisely says that in Austria a seidel was about 0.6229 of a pint, which is about 295 milliliters. Other measures have it from somewhat less than to much more than a quart.
H.L Mencken knew what a seidel was, as well as the full measure of his fellow men: “The average man, at least in England and America, has such rudimentary tastes in victualry that he doesn’t know good food from bad. He will eat anything set before him by a cook that he likes. The true way to fetch him is with drinks. A single bottle of drinkable wine will fill more men with the passion of love than ten sides of beef or a ton of potatoes. Even a Seidel of beer, deftly applied, is enough to mellow the hardest bachelor. If women really knew their business, they would have abandoned cooking centuries ago, and devoted themselves to brewing, distilling, and bartending.” (From Prejucides: Second Series, 1920, New York: Knopf.)