No length limits in principle.
Tasks fit that, and subtasks can too.
A particular machine, doing a particular task, is the TM in question. Usually when you are talking about TMs, you mean UTMs, because you mean a device that can do any computation, and not a device that is just hard-wired to do one particular computation. Think of a TM as a dedicated computer that can only do that one task. It does it the same way any computer, TM or UTM, does it, namely by symbol manipulation, etc., but a TM can only do that one task, the UTM can do any.