As the general definition of a legally significant quantity for Mitzvot/Transgressions involving amounts of food: Yoma 80a, Eruvin 4b; Succah 5b-6a; Pesachim 49b-50a
As the legal minimum of a prohibited item one ingests, to require a Chatat [sin offering]: Yoma 80a
Only considering an olive eaten within the time it takes to eat a "Peras" to be like eating an olive-sized amount, biblically or rabbinically: Pesachim 44a, 114b
Applying the above-stated rule to mixtures containing Leaven: Pesachim 44a
The Olive-Size is determined by a medium-sized olive: Yoma 80a
Swallowing an olive in a big gulp: Pesachim 44a, 115b
A person's eating must match the norm: Pesachim 44a
Power of human courts to change the definition of an "olive-sized amount": Yoma 80a
An olive, liquefied, produces a "Reviit" amount: Shabbat 77a
Size of an olive in relation to a date: Keritot 14a
The maximum number of olive-sized amounts that the throat can hold at one time: Keritot 14a