This has been explained counteless of times on the forum, and of course is clearly explained in the manual.
GSX IS NEVER WRONG when it says a parking is too small, because it READS what YOUR scenery AFCAD is telling. If the parking radius of the parking spot you selected, as defined in the scenery AFCAD, is smaller of half of the wingspan of the airplane you are using, it means the parking IS too small for your airplane, so GSX will obviously report this.
Of course, GSX has to believe what the AFCAD is saying, because it has no knowledge of the *actual* available space in the parking spot, it doesn't know anything about nearby buildings or other obstacles, the only information GSX can use is the AFCAD. Since GSX can't possibly know if the AFCAD is correct and that size is really the maximum usable size, or it's simply a bug of the scenery, it doesn't have much choice that refuse the parking.
The Scenery editor that comes with GSX, allows you to OVERRIDE what's inside the AFCAD, without having to modify the AFCAD itself. You can specify for a parking, a group of parkings or even the whole airport, the spot size that GSX will check against. There's a little "Airplane" icon, that will automatically set the spot size for the airplane you are currently using.
Of course, all of this is clearly explained (on top of being asked and answered too many times on the forum) in the GSX Manual, at Page 24, the chapter aptly named "Max Allowed Wingspan"
