AusBonsai

I have an opportunity to air layer an older Japanese Maple, not what I expected to come across this far north. It has just come out of dormancy in the last couple of weeks and is growing strongly. The question is what size layer should be best to attempt. The main branches are around 50 - 60 mm, would they be a viable option or would it be better to concentrate on this years new growth?
These are examples of the material not the intended layers.

One of the advantages of layering is that we can strike larger/older wood because the tree keeps the layered area alive until it forms roots.
I've layered 30-40mm thick branches but have not personally done any as large as you are planning but should still be possible.
Having said that, younger wood always seems to root quicker and more reliably.
I'd probably hedge my bets and try a few of each.

Just FYI: I've never actually seen a great bonsai from a layer off a landscape tree. What looks great from the ground always seems to be too straight/thick/no taper/big scars when it finally makes it into a pot. My advice is to be very, very selective about where you choose to layer.