I use
irfanview to resize images. Download/install it. Open it. Press b to open the batch convert window. Use the browser that pops up to navigate to the folder with all your frames. Click add all. Browse to where you want to store the resized images. Click "Use current ('look in') directory"
Under "work as", click "batch conversion". Output format, png. Options, compression level 9. Uncheck Save Transparent color unless you want to select one for EVERY frame.
Check Use advanced options. Click advanced. Check resize.
Now, 1920/320=5.053, and 1080/180 = 6. This means you'll never get true full screen on your resolution with perfectly square pixels. If you really, really want full screen to the exclusion of others who may view your video:
If you want perfectly square pixels, resize by %500 for both height and width. You'll either have some slight blurring if you play it full screen, or a border if you play it windowed.
If you want full screen, resize by size, width: 1920 height: 1080.
I would recommend resizing by 200% for height and width if you plan to distribute it, though. This will fight random guy's video player upscaling at least a little better if he wants full screen, while also keeping it likely he'll avoid the same sort of thing affecting him if he wants to make it smaller for his screen. (300% might make it too big for someone who wants to play it windowed, and 201-299 would result in not square pixels.)
Uncheck Use Resample function so it doesn't blur when it upscales. Everything else should be unchecked. (If your images come out with an odd palette, check CHANGE COLOR DEPTH:, select 24 BPP and convert again)
Click okay, start batch. It will create all the resized pngs in the folder you selected with "Use current ('look in') directory". Drag those into Virtual Dub, and make the AVI. (Just for the record if you do make a true-fullscreen-for-your-res-AVI, it will likely be huge. With 200%, it will still be huge, so have at least a few gig on your hard drive free just in case)
The fact is that if you let the video player resize it, whether it makes it bigger or smaller it may always look a bit odd.
Let me know if this goes any better. If not, I'll see if I can think of anything else that may be going on.