AusBonsai

I can agree with Asus to some degree about letting the Chinese elms grow in the ground. I have Trident maple and Chinese elm grow wild in my yard from surrounding trees. The tridents are very easy to remove but when I went to remove the elms there was alot of grunting/swearing as the stubborn buggers would not come out.
These where growing where they landed though. Beside pathways and such. Now I tend to pull them like weeds instead of leaving them for a year or two (just in case I want them later ) as they can be a real bugger to get out. But I see no reason not to plant them out in a dedicated grow area

When is a good time to take chinese elm cuttings and is there a maximum thickness that will root well?

I find good results from cuttings in late winter or spring - early summer.
Theoretically no maximum size. I usually take cuttings around pencil - finger thick but last spring I struck a 3 cm thick seiju cutting. The real problem is finding a well shaped piece that thick - most will be straight which is not good for bonsai.
Note that cuttings may not root. layers are far more reliable with thicker material.

Thanks Shibui