I would recommend 4 meters wide and 2+ meters deep at least. Trolls can still smack with a log across a 4 meter trench so keep that in mind as well with how you build your outer walls and defenses.

another tip that might be handy, keey an air gap between any stairs/walkway and the inside edge of your trench. Other than blobs and related most mobs can't jump at all and even a small air gap It will prevent the mob's AI from being able to plot a path to you or even get to you if the can see you.

I use a single 2 meter wood beam on the inside edge of my trench as a spacer and build the walk way off it until it reaches the outer edge. Then I break the wood beam, the resulting air gap is small enough to walk over but enough to break the mob's pathing AI.

The only mobs that you will need to worry about with agood trench around your homestead will be Trolls throwing rocks, Drakes flying over it, and Blobs jumping it if you don't have a good wall as well. And it's a good idea to upgrade your outer walsl to stone as soon as you unlock the stonecutter so save all that stone you get.