Paper collections should be measured via the direction of paper being filed. Generally, that is the width or length of a box, but things can get tricky, especially with a flat box.
In general:
5" doc box = 0.42 Linear feet (LF)
Records carton (letter size) = 1.25 LF
Records carton (legal size) = 1 LF
3" high flat box = 0.25 LF
When in doubt, ask the Archivist.