What Size Lavender to Plant?

What would be the minimal size of lavender plant to put directly into the ground at this time in northern California (zone 9)?

Size of plants does affect survival. Generally, the larger the plant the greater the chance of surviving winter cold or summer heat. But, as is so often true in the plant world, there are exceptions and limits. If the plant is too big it will have a harder time adjusting to transplant shock than a smaller one would. And old plants are often more susceptible to disease and general weakness than smaller ones.
A critical decision concerning Richters plants is whether you should buy plugs or potted plants. Our plugs and pots are both young so the question of whether they are too large or too old plants does not apply. However plugs are by definition smaller and more delicate than potted plants and need more time to get established in the garden. If they have not sent down new roots and sent up new foliage by the time winter sets in they may not make it through the winter. Therefore, my preference for late-summer or fall plantings is potted plants instead of plugs. It's not that plugs can't succeed, but the chances of winter losses are greater.