I believe that version's crop tool supports a typed-in size for each side and while you're selecting the region, it will treat it as an aspect ratio. Upon completion it will resize what you selected. If not, then I'm pretty sure the rectangular lasso will do that. (Aspect ratio)

Say you want a 200 x 200 thumbnail. You want any selection that is square, so you use whichever tool will let you type a 1x1 ratio. As you select, the box will remain square.

Or - if you wanted a 300 x 200 thumbnail, you would use 3 x 2 aspect ratio.

Note, however, that I think the crop tool will let you type the final size you want. Only if it doesn't, you can use aspect ratios, and then resize as a final step.

Resizing is easy - use Image -> Image Size. Enter the size you want in pixels, and make sure bicubic resampling is enabled.

Finally, use File -> Save for Web. Select JPG and adjust the quality to stay under Rendo's 30K byte limit.

If the cropping or selection tool you're using does not allow restriction by aspect ratio, but instead does restriction by exact pixel count, then it's a little more of a pain in the butt. You have to guess what size rectangle to select/crop up front which might take a few tries. Once you've positioned the region you want, you crop. Then resize to final size, and save as JPG.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)