Here's an easy way to visualize 16 to 18 cm: The rim at the top of any 12 ounce soda can like coke, 7UP, etc. is very close to 17 cm in circumference , so use that as a reference. In terms of inches across or diameter, that works out to about 2 and 1/8th inches, or a little over 2 inches across. Now, that sounds like you'd need a pretty big bulb to match that, but you don't. Try it sometime and you'll see. Most all bulbs you'll receive will be larger than that, even. Except, of course as Rick says, Species sizes should not be compared to commercial hybrid sizes. But even then, the sellers should describe the approximate size in some fashion.
