well, for trick boards, the best length is right around three feet (depending on width)
width should always be able to cover your feet so ur not hangin off.
another good judge of length is that it should be long enough that when u hav ur back foot on the tail, and front foot on the screws in frontside of board, your feet will be slightly more than shoulder length apart, so its big enough for comfortable riding, but not too big and heavy for tricks
inches are the way to go for the deck. for trucks you should use centimeters (truck width). for wheels, you should use millimeters. if you are measuring a longboard deck (full size longboard) you can use feet and inches (since no board is a perfect 3, 4, 5 feet, etc...)
Centimeter
A centimetre should suffice.
'Foot' or 'meter'. Your choice, depending on your preference and the audience.
The head or the point of the pin? I would measure the head in millimeters and the point of the pin in micrometers.
A standard board is usally 31-33" long. Usally the shorter the skater the shorter the board. The lenght of a skateboard is personal preference. Over time a skater will find out what size board works best for them. It is kinda hard to go wrong with length any standard board is appropriate.
It is still customary to measure, in inches, a screen from corner to corner diagonally.
What is the most appropriate instrument to measure
The width of a highway would be best measured in meters. The length would be measured in kilometers.
inches
Meters
A tenth of a metre.
feet