A size 18 needlepoint canvas would be used for small projects and/or where the design is very detailed. It will take longer to work a size 18 mesh canvas than it will a size 10 canvas of the same dimension. For a beginner learning how to needlepoint, size 10 or 12 mono needlepoint canvas is a good place to start.
The size of needlepoint canvas you choose will also depend upon the thread you plan to use. The thread should glide smoothly through the mesh. If the mesh is too small it will shred the thread. If the mesh is too large the thread will not cover the canvas and the canvas will show through. Not a good look!
So when choosing a mesh size you need to consider not only the design you want to stitch, but also the thread you plan to use.
Interlock needlepoint canvas is also single weave, like mono, but the threads are twisted together which makes the intersections a bit more rigid - harder to accommodate a variety of stitches but fine if you are stitching a continental needlepoint stitch. There are different mesh sizes of interlock needlepoint canvas, just as with mono canvas.