Longboards made for cruising and carving are typically 38" to 60" long, with some being as large as 80".
This board is 38'' This board is 80''

The length of a board serves different purposes. Smaller boards will typically be easier to maneuver yet will always be slower. Smaller boards from 38" to 50" are great for carving hills, which is the act of executing smooth, controlled turns. Carving on a longboard is very similar to carving on a snowboard.
CARVING!

Longer boards are great if you are riding on a lot of flat land since a couple of pushes will propel you for a long time.
This board is wicked sweet

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