Answer
What size boat slip do I need?
Pick a slip 2–4 ft longer than your boat's overall length, with width that exceeds your beam by at least 2 ft. Confirm depth at low tide matches your draft plus 1 ft of clearance.
Length
Slip length should exceed your boat's LOA (length overall, including pulpit and swim platform) by 2–4 ft. A 38 ft boat realistically needs a 40 ft slip.
Width
Confirm the slip's width between finger piers exceeds your beam by 2 ft for cleat access and fender placement. Catamarans need T-heads or end-ties for their wide beam.
Depth
Controlling depth at MLW (mean low water) should exceed your draft by at least 1 ft. Tide swings of 6+ ft (Maine, Georgia) need more cushion.
Bridge clearance
Sailboats and motoryachts with tall arches must confirm air draft against any bridges between the slip and open water.