Gorton Radiator vents - physical size?

I am looking at getting some Gorton air vents for my radiators. I'm hoping they will help with some spitting that is going on with some heat-timer varivalves I have. One or two of these are going on baseboard steam which means I have limited space for the vent (due to the baseboard housing), hence the varivalve which is pretty small.

So, the question:

can someone tell me the physical size of the vents? Gorton's site doesn't seem to have that info. Are they the size of a quarter or half dollar or bigger?

Thanks

Tom