Filmformat 16:9 or 2:40 ?

I have often worked with the format 16:9 but now if you want to have a format it´s more something like 2:40 ( something)?a

I can´t find a format like this to choose from in After Effects ( I have 5.5
There is something I haven understood I think.

Some help in this issue woulb great.

/ Mattias