Ive been using Strata 3d for many years and have been used/spoiled with this feature. In Strata you just select an area, from any window, to render, and with the Shift Key selected, the render output settings shows up so you can make your selection of the scene, any size or rendered with any of the rendering options.
Now that I have switched to Cheetah this would indeed be a nice feature, and timesaver.
I was wondering about that also but in the mean time, we can simply rend a tiny test render like 360 by 240 which is quick enough.
Yes, a test-render 'selection window' would be great.
Sometimes I want to render a test for something that's at the bottom of my scene and have to wait for the whole window to render.