The conventional wisdom is that you should size clutches so that the clutch's max line size is as close as possible to the size of line you're using. E.g., if you're using 10mm line, get a clutch with a max line-size of 10mm. The wisdom is based on strength being related to how much of the pressure-plate is used.... so a line that uses all the surface area of the pressure plate will get the most from the clutch.
Having said that... if there's ever a CHANCE that you'll use larger line, it may make sense to consider a clutch able to take the LARGEST line you'll use, because you won't be able to use it in a smaller clutch.
Note that some clutches (eg, Spinlock) have the ability to use different size plates, so you can convert it from (for example) 4-8mm to 6-12mm.... same clutch body, just different parts. I don't know if you can do that with the Lewmar, worth asking.
Bruce