Of the mk2 Sierra type there are actually quite a few different sizes. I did a calculation of what the stiffness is supposed to be based on this formula:
14 mm - 7.96 N/mm (Sierra XR4x4)
16 mm - 13.59 N/mm (Sapph Cosworth 2wd)
18 mm - 21.76 N/mm (Sapph Cosworth 4wd / Granada 24v)
20 mm - 33.16 N/mm (bug-eye Scorpio)
21 mm - 40.32 N/mm (bug-eye Scorpio 24v)
22 mm - 48.56 N/mm (Escort Cosworth)
As you can see the stiffness goes up very quickly with the diameter. A 3dr arb is also 14 mm thick, but is a lot longer than the later type arb so will be quite a bit softer. At a guess it might only be something like 5 N/mm.
I have a 21 mm Scorpio 24v rear arb on both my cars. I also used droplinks with rosejoints which give quite an increase on the arb stiffness as well, as the rubber in the standard droplinks is quite soft. What size is best depends on the rest of the suspension, so front arb, springs, shocks, etc. So it really is trial and error. But I'd say that around 20/21/22 mm is a nice size. Of course it also depends on what you personally like. When it is about being fastest on the track a softer rear arb on a 2wd car might actually be better for getting the power down.