You have a couple of problems there. First, you only need quoted form for shell scripts; you shouldn't quote the path here. Second, you're building a URL called anNSURL, but then you try to pass one called myURL. And I think you'd be better to take that stuff outside the QuickTime tell block. Try this:
use scripting additions
use framework "Foundation"
use framework "AVFoundation"
tell application id "com.apple.quicktimeplayer"
activate
set original_file to get original file of document 1
end tell
set posix_file to POSIX path of original_file
set anNSURL to current application's NSURL's fileURLWithPath:posix_file
set theAsset to current application's AVURLAsset's URLAssetWithURL:anNSURL options:(missing value)
set theTrack to (theAsset's tracksWithMediaType:(current application's AVMediaTypeVideo))'s objectAtIndex:0
set theFrameRate to theTrack's nominalFrameRate()
set theSize to theTrack's naturalSize()
return {theFrameRate, width of theSize, height of theSize}