Warning.
If you do trade speed with length, you DO change ships speed (try this - change it to 500, open mission editor and you will see that you can set speed of that ship up to 500 kts! (speed max=500 kts). Now, that could be the problem if game has scripted (go to max speed) instead of (go to speed=32)...
I have tried to add length and aspect ratio to rec. manual with 1024_768.ini file.. no luck. I can't add new Item ID...
Insted, I have found temp solution