The most popular line sizes used for tenkara are 3, 3.5, 4 and 4.5.
The diameters are as follows (conversions to inches are rough and subject to rounding error)
Line size - Diameter
3 - .285mm - .011"
3.5 - .310mm - .012"
4 - .330mm - .013"
4.5 - .350mm - .014"
5 - .370mm - .015"

Tenkara line packages do not list breaking strength (since the light tippet is always what breaks first, to a Japanese tenkara angler it would be like asking what is the breaking strengh for a 4 wt fly line - who cares - it will never break). However, because you can use fluorocarbon spinning line for tenkara, and in the US at least, it is sold by breaking strength, there is a reason to ask. The rough equivalent is to take the Japanese line size and multiply by 4, so size 3 line is roughly 12# test. Size 4 is roughly 16# test.