Hi AnnMarie,
Beads are much easier to measure using a measuring gauge, than a ruler. I think that most hardware stores carry measuring gauges, but I'm not sure. Some craft shops may also carry them. I'd recommend calling first, and asking.
Here is a link to a picture of a measuring gauge: 65-100. Click on the tiny picture to get a larger version. By Tuesday, we should have an "extended description" built into that stock number, with a hint for reading a measuring gauge even more precicely.
You can measure beads with a ruler, too, but you can really only estimate. When you set a ruler next to/ above/ under a round bead you can eyeball it and guesstimate that it is about 6mm (millimeters) wide, or whatever size. It's hard to guesstimate with more than a millimeter worth of accuracy though.
With a measuring gauge, you slide the gauge open, then clamp it down on the bead, and it tells you it is exactly 3mm, 4mm, 6mm wide etc. (and if round, then it is also 6mm tall, and 6mm thick).
An oval or tear drop bead might be 6x4mm, 10x8mm, etc.
A flattened round bead might be 6x6x2mm.
In our catalog, we always try to measure Height x Width x Thickness. To determine the "height", we hold the bead the way it will hang it its picture.
I hope that answers your question!
Please let me know if you have any further questions.
--Polly