While I was rendering quite huge image (3636 x 2657) for Master & Servant challenge I found out that I wanted to render just a portion of image to verify if polycount is smooth enough in particular areas. For viewport usage there is this handy tool "Render Region" which is nice but with it I can only render low resolution version and to a modeling viewport only, not to a renderview or to a file.
Instead of rendering whole image just to check if everything is ok would be too time consuming, since every render took about 2 hours, so I ended up making this little mask which helped.
1. Make a foursided polygon. For example create Plane -object, change rubdivisions to 1 in x and y dimensons and convert it to a polygon object (hit C on keyboard).
2. Go to select polygons -mode and select this polygon.
3. Use "structure-extrude inner -function" and scale so this new polygon is at least 50% smaller than original.
4. Hit delete from keyboard to delete this new polygon, so you end up with a rectangle which has a hole in it in the middle.
5. Give it a render tag and disable everything but "Seen by camera".
6. To be really sure it's not affecting or get affected by lights create new material and activate only a luminance channel and change it to a pure black.
7. Bring it very near of camera-object (I used Transfer-function from Functions-menu).
8. Rotate it so that it faces towards scene-objects and move it a bit nearer to a objects but make sure that the outer sides of "mask" covers whole viewport area.
9. Scale and move it to reveal only a part of your scene you just want to render. With point-mode scale only the inner hole if needed.
10. That's it! Hit "Render to Picture viewer". Image is totally black except from area which hole "covers" and part of your scene is shown there. Rendering is fast on black areas so you can test render specified areas of huge images quite fast.
You can even render several different images where this "mask" is in different positions and combine those images together in imagemanipulation program. Another nice thing is that if just a portion of your image is wrong you can render just that part and combine it with original image by deselecting black areas.
Here is a screenshot to help if necessary.