[arthurcolle]

Different derivatives have different lot sizes iirc. Options contracts for listed equities have a lot size of 100. Once options markets became more standardized in 1973 with CBOE introducing call options, the default lot size became 100.

From /u/ins2be on reddit:

"That's what the OCC sets it at in their bylaws.

Unit of Trading

(5) The term “unit of trading” in respect of any series of options or futures means the number of units of the underlying interest which have been designated by the Corporation as the minimum number to be the subject of a single option contract or single future in such series. In the absence of any such designation for a series of options or futures in which the underlying security is a common stock the unit of trading shall be 100 shares."