This is ds-sloth, I'm a lead developer for TheXTech and the one who made the new settings system in v1.3.7.
First, please make sure that you're using 1.3.7-beta or a newer version. You can download the current builds for the 1.3.7 release (which should be finalized in the next few days) at
https://builds.wohlsoft.ru/; they'll have the name stable-1.3.7.x.
What do you mean by stretch the game window? If you want it to be full screen, you can set that in the game options menu in the Video section. You can also set your window to be larger by setting the scaling mode to 2x. We don't currently support "stretched mode" where the 800x600 image from SMBX is stretched into widescreen, but you can fill your screen by making sure you are playing at Dynamic resolution (also available in the Video Settings).