Has anyone here ever threaded a sten barrel for a suppressor?  If so, please advise on size of die.  I have never threaded a barrel

Thank you

can be done if you have the equipment pretty straight forward

KAK did a run of them awhile back in different lengths and thread pitches the one I tried ran great, if you want to keep your original intact

barrels

Using a die to thread a barrel is generally a bad idea. First any barrel you buy will need to be turned down to proper diameter for the thread size that will be cut. This must be done on a lathe and be perfectly centered to the bore of the barrel and provide a square shoulder. Second using a die to cut the threads will usually results in non-concentric threads, not what you want for a suppressor mounting point.
The only way to properly thread a barrel is on a lathe by single point cutting. If you don't have a lathe then you will need to have a gunsmith do this or purchase a barrel already threaded for a silencer.

Red, agreed.

Green.... Not so much.  See also die heads and rollers.

Yes you can get satisfactory results using a die but it takes a very good setup and is much more difficult to control thread fit/class. Single point threading is the gold standard for manual lathe work and widely used by gunsmiths for a reason.