Valve sizing

[turbodave]

OK guys, time to show my ignorance again. http://www.turbobuick.com/forums/img/smilies/rolleyes.gif

I have this really nice pair of SII aluminum heads (bare). I would like to start looking for some valves. What's the best way to measure the seats so I know what size to ask for??
Also, what stem length and size would they take?

[Intercooler]

Off Topic..... Did the magazines show?

[turbodave]

Off Topic..... Did the magazines show?
Yes, thank you!

I was hoping someone could give me some hints on how I might use my calipers to measure across the valve seats to estimate what size I could use.