Peter Piper picked a peck of pickled peppers, but just how much is a peck?

A peck is an imperial and U.S. customary unit of dry volume, equivalent in each of these systems to 2 gallons, 8 dry quarts, or 16 dry pints. Two pecks make a kenning (obsolete), and four pecks make a bushel.

https://en.wikipedia.org/wiki/Peck

"A peck is an imperial and U.S. customary unit of dry volume, equivalent to 2 gallons or 8 dry quarts or 16 dry pints. Two pecks make a kenning (obsolete), and four pecks make a bushel.

In Scotland, the peck was used as a dry measure until the introduction of imperial units as a result of the Weights and Measures Act of 1824. The peck was equal to about 9 litres (in the case of certain crops, such as wheat, peas, beans and meal) and about 13 litres (in the case of barley, oats and malt). A firlot was equal to 4 pecks and the peck was equal to 4 lippies or forpets.

Conversions
1 imperial peck equals:
9.092 litres
307.443 U.S. fl oz
320 imperial fl oz (exactly)

1 U.S. dry peck equals:
8.81 litres
297.894 U.S. fl oz
310.061 imperial fl oz

1 U.S. liquid peck equals:
8.571 litres
256 U.S. fl oz (exactly)
266.456 imperial fl oz"
-from http://encyclopedia.thefreedictionary.com/Peck

When Peter Piper picked his peck of pickled peppers, he picked the equivalent of 1/4 of a bushel. While no one knows the origin of this word nor how it came to be a unit of measurement, we do know that Peter's peck of pickled peppers amounted to the equivalent of 2 gallons of dry weight, or 10 to 14 pounds.

https://forum.wordreference.com/threads/pick-a-peck.2054479/