I've never had much luck with propagating anything from cutting before so already feel like I'm pushing my luck...! However I've recently bought a house with a cotoneaster bush and would like to attempt to bonsai it and have a few questions:

What is the thickest cutting I can take that will still root? What is the best time of year in Scotland to take a cutting? Would it be beneficial to try and propagate outdoors where it will get more sun but will also be susceptible to the four-season-in-an-hour weather here, or to grow indoors where the light is somewhat limited in winter, but more consistent temperature, etc?

As its my own bush now I plan on taking a number of cuttings in the hopes that something takes however the above will go a long way to making sure I'm doing things right and not setting myself up for failure by making a silly mistake.

Thanks!

Always propagate outside

Instead of cuttings, take a look at airlayering. Is has a very has rate of success.

The largest cutting.. With cotoneaster shrubs should be fairly high, as they root very easily. I would however for sizes of over a cm or so always opt to airlayer instead.