I like small projects and weavings that have an end in sight... LOL!
I think my most versatile size is my 18". Even after fulling my wool
weavings, the finished triangle is large enough to make small things with on it's own,
but it's also small enough to work well for piecing triangles together very nicely.
I have one that's a size smaller... about 14 inches if i remember right... I like that one because it weaves up so fast and when I full down the wool triangles they are sweet little "gems" that can be put together for all kinds of items.
Some day I WILL build a large shawl loom... but for now I don't have the space for one and I don't have the ambition for setting all those hundreds of pegs. The next loom I build is going to be a small square continuous weave for dishclothes and such.