All text should be legible from 3 ft. Use only high quality, clear graphics. If you really want to show something and can't find a good slide or graph, substitute a line drawing. In general people put too much information on posters. It's better to use the poster to draw audience in and then have handout for them to take away that provides additional information and a contact person.
Handouts are also easier and less expensive to update as new information comes available.