You need to edit the xml file of the plane (on PC they're stored at "C:/Users/YourUsername/AppData/LocalLow/Jundroo/SimplePlane/AircraftDesigns"... I think... I'm on mobile right now so I can't check)
I prefer to save the design, and then edit Editor.xml so I don't lose my design if I mess up.
Just open it in any text editor (unless you wanna get an xml editor, for highlighting and to reduce mistakes), find the fuselage you want to edit (the PartType tag closest to the bottom is the last one added, but if I don't know when I added it I give it recognizable/unique dimensions in SP editor and search for those dimensions in the xml), and change it's... end size? I forgot what variable it is... just experiment, it's got three numbers I think, like "1,1,2", and there is one for each end of the fuselage.

Edit: grr... where's the formatting page on this site? I need to escape it for editor.xml which has three (_ ) on both sides of editor...
Also, I see Skua beat me... I'll leave this here for some detail, but you'll need to figure out more detail anyway

If you're on PC or Android you can edit the save files of your planes with a text editor, usually referred to as xml editing or modding on this site. This lets you type any size you like, along with a bunch of other stuff like changing the maximum thrust of engines and positioning parts in ways that are otherwise impossible. Users on iOS can't usually do this because Apple hides the game files away. What you can do is save parts from other designs as subassemblies, or request them here on the forums if you can't find anything suitable.

One cannot. One must hack the game