if you're only interested in the size, try to look up the statistics of the engine (what you'll be needing is "dimensions", these are things like the length, diameter, and weight — if possible, look up official papers on it, but plain old Wikipedia might work too), try to match those values inside the game by resizing the engine (note that you may need to use the hidden properties editor to change it more precisely), in one of the panels to the right the game will display your craft's dimensions in metres (height, length, width, + more), and make sure you make your control unit a very small command disc that you can then move inside the engine so that it doesn't add on any extra meters, after you're done with matching the size (note that the performance will likely not match, it is much more difficult to change that for jet engines unfortunately) you can save the engine as a subassembly or something (if you're planning on using it on a plane)
if you're only interested in the size, try to look up the statistics of the engine (what you'll be needing is "dimensions", these are things like the length, diameter, and weight — if possible, look up official papers on it, but plain old Wikipedia might work too), try to match those values inside the game by resizing the engine (note that you may need to use the hidden properties editor to change it more precisely), in one of the panels to the right the game will display your craft's dimensions in metres (height, length, width, + more), and make sure you make your control unit a very small command disc that you can then move inside the engine so that it doesn't add on any extra meters, after you're done with matching the size (note that the performance will likely not match, it is much more difficult to change that for jet engines unfortunately) you can save the engine as a subassembly or something (if you're planning on using it on a plane)