The easiest way is to use aircraft radius
You need of course AFCAD and AITM which you should have already, if not they are freely available.
I have made up a list of Aircraft that I use and set radius in feet and metres. Reason is that AFCAD is in feet and AITM in metres. For example 737-400 for one airline I use 15 metres/ 53 feet
When you make a parking space using AFCAD put in the appropriate radius in feet and save.
Then go into AITM aircraft editor and for the same aircraft put in the radius in the box for radius in metres and save.
This will make this aircraft park in the closest radius parking and it works well.
The only hitch is that if you use the same aircraft with different airline paints the AITM will only use one radius.
So in this case look for a another Aircraft download if there is one  available, usually is for popular fleets,and i you can set it up  to a different radius.
I have over 40 radius in use and they generaly work really well.
I have heard of some standardisation for radius and maybe it will come with the next FS
Whatever radius you use and as long as you set these two the same it dosnt seem to have any tie up with the physical size of aircraft.
Good luck