Bigger bulbs produce more flowers the first year after planting. Smaller bulbs will eventually grow under favorable conditions. Lily bulbs that are too small (less than 12 cm in circumference) will not bloom until they reach that size.

Alina is right, except one must remember that different lilies 'mature' size is different, so some bulbs will be naturally smaller but still mature.

Thank you very much. Is 18cm considered a large bulb?

For what variety? It is a fairly large for Orientals and Asiatics, but regular size for Trumpets for example.

I'm not sure. They are listed as heirloom and chinese trumpet. The Heirloom ones are rubrum, black beauty and album. Thank you very much for you help and knowledge!

I'd consider 18cm as an average sized asiatic or oriental lily bulb....not especially large. This size is pretty common for better mail order bulbs. Aurelian hybrid or trumpet lilies typically generate larger bulbs. A local lily grower (and one of the better mail order lily sources) offers what they call 'field harvest' bulbs to their customers locally and these are massive things, sometimes the size of a small cantelope and weighing several pounds, but these are never shipped and are only available at their local outlets. The attached link is how they explain the correlation between bulb size and plant performance.

Here is a link that might be useful: is bigger better?

Interesting article, Gardengal. Good explanations about comparing sizes in terms of volume instead of diameter/circumference, but the statement that 24cm bulb is 71 times "better" than 10cm bulb has to be taken with a grain of salt, to say the least ;o)
BTW, I was not happy with my last year order from the Lily Pad. Some of the bulbs had basal rot.