Power DVD can give either original video size or Fullscreen size of caps. I don't think VLC does fullscreen caps, only original video size.
If you go to preferences in VLC (ctrl-p I think) there's a choice between 'simple' and 'show all' (just guessing here, this thing is in dutch) in the bottom left corner.
When you tick the 'show all' box, you can click 'video' and then you will be able to manually set the width and height of the screenshot, set in pixels.
Click save, exit, reopen and you will have screenshots in the size you want, but make sure the expanded width and height is similar with the original size (320x240 to 640x480 and so forth) or else the image will stretch out.