Pony bottle size

[khacken]

I am thinking about buying a pony bottle as a bailout air source. I don't plan on doing anything technical (always 130ft to the surface). What would a good size be? I get the feeling that the spare air bottles with 3L is too small.

 

[khacken]

Found the answer. I am going for a 19

http://www.scubaboard.com/showthread.php?t=130446

 

[khacken]

Mod, please feel free to delete if you feel this is a repost.