So takeshita, you’d just use the first method…b for the square then select. However if the only verts you want form a true loop your can also push alt+rightclick on a vert in the loop and it will only select those verts.
You can use the + and - key on the numpad or the - and = key on the “normal” keyboard to grow or shrink the circle, if you don’t have a mouse wheel.
Martin