I would not prune it too severely now. I would do it in easy stages. If you give it a light prune now it will stimulate new red growth which is attractive. And then next spring or early summer after the normal flush of new red growth I would give it another trim. I dont know by how much you want to reduce it but in easy stages until you get it the size you want is probably best.

I would not trim it in winter but do it while it is in active growth. Too sudden and severe a pruning at any time might give it too much of a shock and you may just possibly lose it.

Inverglen is quite correct. It is also used as a hedging shrub so you can see it can be trimmed without problems, at the right time. To keep the red leaves you will need to trim.