Just as in Terragen, you can set the size of your render to be as large or small as you want. Also, you can specify file type. For an easy quick thumbnail, one way would be....go to File > Document Setup, set the size and ratio that you want (say 100 x 75) render your thumbnail, then File > Save Image As, and select the image-type from the drop-down Save As Type choices. There may be better ways, that's just the way I'd do it if I didn't have Photoshop :o)