UPDATE: Make sure and read the comments. Kees and I have an interchange on the subject of high bed angles and chip breakers. We discuss there effect on mouth openings. Good stuff. I will let you make up your mind what matters most. Original post follows.
Last year some time I posted plans for a coffin shaped smoothing plane. Of course, to be made out of wood. I asked anyone to point out any errors in the plans. I got some feedback but no one seemed to question the reference to the mouth opening being 1/32". That is quite large actually. It is about double what I would consider as a size to shoot for.
So what is the size I shoot for. I aim for as tight as possible. That means about 0.006"-0.008 is excellent but I am happy with anything up to around 0.012". Now, that is for planes I sell. As for planes that I use, I have some with mouths much larger than that.
Here are a couple of photos of examples of mouth opening on my planes. The first is a smoother at about 0.004" and the second is a miter plane at around 0.010". Click to zoom in.
Of course we are talking about smoothing planes. For jack planes I don't get bent out of shape on the mouth size. About a 0.030 is fine with me or around a 1/32". That is how I think that I ended up with that measurement in the plans. I should be more careful when I copy and paste.
So essentially if you make a smoothing plane with a mouth opening of about 1/64" (0.015") then pat yourself on the back. You did an outstanding job. If you can get it to half of that, then you are ready for a new profession. :)