How do yo measure a Sealed Beam Headlight Bulb?

[BRENT in 10-uh-C]

I realize this probably seems very elementary but how do I determine the bulb size? I have a Coupe in here that has one of those old sealed beam conversation kits that needs a bulb replaced. The one that came out of it is a 6 volt Wagner 2D1 bulb that measures 6.750” across. I am finding Wagner 6006 6v bulbs online that are advertised as 7” bulbs. Is this the same bulb, -or was there a 6.750 diameter bulb also??

TIA!!