Longest flatbed on a 1-ton dually

I am working to fabricate a pickup with a flatbed to carry two 10' SeaDoos and tow a travel trailer. What's the longest flatbed for a 3500/F350? The cab to axle is 56". Any advice will be appreciated.

When considering flat bed length first decide the type of hitch being used. Anything over 8 ft will be questionable if going goose neck or fifth wheel. Rear axle placement will also be a big factor.
Even tho not a Ford my Dodge dually with 9.5 ft flat bed is not goose neck or fifth wheel compatible.
Also bed lengths need to be some what limited to frame length.
Lots of variables and no cut in stone answers.