How do I measure wideness of rim?

[galagmosh]

This might seem as a stupid question, but, how do I measure the wideness of a rim - Do I measure the rim from edge to edge or do I measure from the tire bead areas ?
What i really want to find out is, how narrow a tire can I put on a rim that measures 5.5" from edge to edge ?

[toe]

It's inside to inside where the beads are.

Tire manufacturers usually have specs that state the tire width on a given rim and the rim sizes that work for that tire...