OK, so i measured a standard 3.5 inch spoon that has 2 split rings and a treble hook. i got 5 3/16- 5 1/4inch.
it can be 100% up to you if you like to pick up spoons every trip after after bumping it around.
I'd hate to give false information in regards to a hassle free setup when it comes to special mate boxes,
i suggest one size up for peace of mind ,
As other here have stated , 3.5 inch spoons will fit in a 5 inch box , but not without a headache of picking spoons up after they get knocked off , due a slight bump. i have 500 plus spoons for walleye, trout and salmon.
I too thought the 5 inch was good, then i talked to the charter guys and saw what they use down my way, ?? there is a reason most use the 8 inch box.
just trying to save you the hassle.
the 5 inch box is great for 2.5 inch spoons up to 3 inch with 2 split rings and a treble.