Garlic is measured by diameter, from one side to the other side across the widest point while looking at the bulb from the top (stalk end) or the bottom (root end).
Garlic growers vary in how they advertise their garlic for sale, so read carefully when looking at web sites.
Some growers consider bulbs 2" in diameter to be "large," while others consider anything over 1 1/4" as large. That's a big difference.
Also, garlic varieties may have differing breaking points between a good-sized bulb and a medium-sized bulb. Some garlic varieties consistently produce 2 to 3" bulbs, while others consistently produce 1 1/4 to 1 3/4" bulbs. Read descriptions carefully if this matters to you.
Each year's weather can also affect bulb sizes. A cool, wet year in the northern half of the US may cause most growers' garlic to be smaller than it was in previous years that were warmer during the growing season.
There is no need to purchase a garlic measuring device.
We've tried a lot of different methods to quickly and accurately measure garlic, and this is truly the easiest. A friend of ours that will be growing garlic with us in 2012 came up with this, and we have really appreciated the ease and simplicity...as well as how fast it is to make a new one when another disappears!
Now, if you want to, you can spend $30 to $80 to buy a varnished board with nails, wires, etc. to help you measure.
Or, you can make a bunch of these for free. :-)
All garlic seed for sale is supplied by our small micro farm in Elgin Oregon - Greifs Gourmet Garlic!