It's 37 linear inches. There are a number of varying rules for personal items. One says it must be 36 l.i. Another says it must fit under the seat in front of you. Yet another identifies a personal item as a briefcase, camera bag or big purse. 36 linear inches is pretty standard for a briefcase.

If you don't overstuff the WF and stow its backpack shoulder straps it should pass muster.

Till