Best size for mobile / iPad?

Hi guys,

What do you think is the best size for mobile and ipad? Should i work in pixels or mm. if i would like to print the design later on? Is there any internet guide to choose the size in pixels depending on most users?

I am doing a Guide of Plants, basically short text and 2 pages of pictures. The client asked me for an interactive PDF (no epub).

Landscape or horizontal format.

PRO
– works right on presentations, tv screen, etc.
– the same size can be easy to print later on.

CONS
– i have to rotate and zoom in in my mobile for best visualization.

Vertical format

PRO
– user friendly in mobile

CONS
– i have to do a new design for printing
– not easily adaptable to horizontal, tv screens formats.

Any suggestion for a project like this?

Thanks for any tips or ideas.
Sebs

ps: sorry about my English