What are sizes of cognac are there?

There are two regular sizes of cognac bottles (I think this is what you had in mind).

In Europe the regular size of a cognac bottle is 700 ml.

e.g. the bottle size of this Hennessy XO Extra Old Cognac.

In the US the regular bottle is 750 ml.

e.g. the D'Ussé VSOP Cognac.