If starting out new to Tenkara or fishing in general, keeping things short and tight will make learning to cast and landing the fish in easier. In general match the length of your line to about the length of your tenkara rod. A 12ft tenkara rod pairs well with an 11ft or 12.5ft tenkara line. Remember you will still need to add a length of tippet to your line which will extend its total length.
As you progress in your tenkara fish or you fish different waters, you may require a longer cast to reach fish. As you gain more expereince you can pair your tenkara rod with longer lines. Just keep in mind that the longer the line the more handlining you will need to do to bring the fish in close enough to net. Also, the longer the line to more effort it takes to set the hook since you are moving more weight across the surface of the water. For more information on lines read this: