It is quite common in games to not use realistic measures. A rule of thumb is, to measure your scene by the main character, in your case your plane. Define your plane and measure other objects by the size of your standard plane. This way you can get a suitable size of your plane first (how does it look from far away etc.) and then create other objects in realation to this (a house should be much larger, therfor the size is 5x plane etc). It could be useful to use the bounding box of your plane as measurement due to the non-compact form of a plane.

Agree with Ashaman73 that you don't need to sweat too much if the measures aren't accurate, but I wonder if the problem might be your field of view. Maybe your field of view is quite wide (which makes things look little) and you've positioned your aeroplane close to the camera to compensate.

If that's the case, then you've effectively done a dolly zoom (google it, watch youtube vids/gifs, they're cool!) on your aeroplane, distorting the background while keeping the aeroplane looking normal. The fix would be to narrow the field of view and move the camera further back from the focus point.

Edit: PS. Wide field of views give more of a sense of speed, so you might want to consider how important that is to you before narrowing the FOV too much.

If the background has nothing to do with the game itself, you can scale until it looks nice.

Measuring is crucial in flight simulations in which fans demand realism, but airplane games often "fudge" the proportions because the average gamer playing them is looking for emotional experience more than realism. The average person is going to be too busy having fun to think about proportions. They are used to distortions in gaming anyway, so if they do notice something then it is typically no issue, but probably they won't care one way or another.