How do I select radiator air vents?

Several of the steam radiator vents in my house are not working (either they don't close when hot, or are clogged). No two are the same manufacturer...

A little searching of heating supply sites shows a bewildering array of vents. The Jacobus Maid-o-mist units seem reasonably priced but there are at least three sizes (#4,5,6)... is there a table somewhere that shows the difference between them? How are they sized?

Thanks! That is exactly what I needed.