Where you live, air layers of larger branches should be started in early spring, though there is some leeway if the plant being layered is genetically very vigorous. Reason: it takes time for enough roots enough to sustain the plant after separation to form, and the layer can't be left over winter, even if you use a pot/soil instead of sphagnum moss as the rooting medium. Too, branches used as propagules should be of the highest vitality on the plant. Weak inner branches or branches on the north side of the plant should be avoided unless they are near the top of the plant where they get plenty of light. How much stored energy the propagule has plays a significant part in determining the probability of success.
There is nothing to stop you from trying, but I'd go with the smaller branches because of timing issues. I do a lot of layering, in case you want some suggestions that will increase your success. Pre-blanching at the layer site (when you have the luxury of time on your side) predisposed the plant to form root initials even before you establish the layer, so you'll likely want to know about that if you plan on going for a thicker branch next spring.
Al
