Vacuum Hose Size?
 

Is there a standard size-diameter for vacuum hoses on a 78' F250 w/ 400ci engine?

There are multiple sizes 1/4 3/8 1/2 I believe

Most will be either 3/16" or 1/4" but some of the larger lines (e.g. PCV) are 3/8"

Power brake booster is half as I recall

Cut off short samples like a 1/4-inch lengths, tally and write down the total linear footage by approximate size, and bring them to the parts store with you to compare to the sizes offered off the bulk rolls.