a meter?
The 'foot' or the 'meter' would both do nicely.
It depends on whether you want to measure its length, its width, its mass, its colour, its density etc.
You can't just convert any unit into any other unit. The units must be compatible - measure the same type of thing. For example, you can convert a unit of length to another unit of length, or a unit of time to another unit of time. An atmosphere is a measure of pressure, a foot is a measure of length, so you can't convert it.
A centimetre.
I say miles that's how the car does it; and kilometres in the SI system.
By unit of length and distance and conversion ,we can say that the most appropriate unit to measure the length of a finger is cm.
Inches
cm
I would use feet
You would use meters.
yards
centimeter
You would use the SI unit meter (m) to measure the length of a DVD case.
You would use kilometers (km).
miles or kilomiters
feet