i would measure it in feet
You can measure a tree with a few things. If I were you the easiest thing would be a ruler.
kilometers
You would use meter's to measure an oak tree's height.
A flexible tape measure is best. A string will do also. Just wrap the string around the tree until it doubles up on its self, then measure the string.
To measure the height of a tree accurately, you can use a clinometer to measure the angle from your eye level to the top of the tree, then measure the distance from the tree to where you are standing. By using trigonometry, you can calculate the height of the tree.
Definitely in meters.
Usually meters would be an appropriate unit.
Your question presumes that tree trunks are all circular. If you happen to find a circular tree trunk, you would measure around it to find the circumference. If the tree trunk is not circular, you will not be able to find the circumference but, you can measure around it to determine its perimeter.
Your best bet here would be to use trigonometry. One stand a given distance away from the tree and sight the top of the tree, say with a clinometer, to measure the angle of elevation. Then knowing the angle and the distance you are from the tree, the height of the tree can be calculated with a tangent function.
Meter, since I've never seen a tree that was a thousand meters high. Where do they find you people?
Surely if you have the tree you are the person to measure it and let us know. Surely if you have the tree you are the person to measure it and let us know.
Feet or metres would seem the most suitable.