Hi there,

I suppose they question is whether you merely want to extract a 800x600 pixels segment from your 1024x684 video clip and make that you full screen video, or whether you actually want to export some video file that is 800x600 pixels, which would be a somewhat uncommon format for a lot of video applications. But then again, I don't know exactly what you had in mind and what you wanted to do with the resulting video. I guess one way or the other, this is how you would start out:

Import the source video file into the program and open the size & position editor on it, so you're basically in the screen shown in your screen shot above. Under the "Clip" section, put in the following values:

Left:     224
Top:     0
Width:  800
Height: 600

That will restrict the selection to the part you want. Now, while you're in the "Object size/position" mode, hit the "Maximize" button. Confirm the image size & position dialog by pressing OK. Done. Your chosen 800x600 segment from the source material should now fill the entire video screen.

What the actual size of your video is now depends on your project/movie settings, of course. By default you're probably operating in standard NTSC or PAL resolution, which is of course not 800x600. You can manually force this size, though. This can even be as late as during the export stage: Go to "File" => "Export movie" => the desired output format, e.g. MPEG. In the export dialog, manually change the output resolution to 800x600 pixels and proceed with the export. Override the warnings you may get. This will result in a resulting file with the actual dimensions of 800x600, if that is what you were looking for.

There's probably some better way to go about this and maybe this is not exactly what you wanted to accomplish, but hopefully this will give you some ideas how the size & position dialog can be used.