Currently, when I try, it's either fullscreen, covering my widescreen monitor which looks ridiculous, or WIndow which is about the size of an iPhone screen. Is there a registry setting that can be tinkered with to increase the Window?
No need for registry editing. If you use Galaxy, then right click on the game, go down to Other, and choose Graphic Mode Setup. In the window that pops up, select the Advanced Settings tab and you will see an option for Window Resolution that you can select from.
If you don't use Galaxy you can use the same program. Go to the folder that the game is installed in, and double-click the shortcut for DOSBox Configuration. It will load up the same thing.
If you want to manually tweak the settings through Notepad++ (or some other text editor), you can open up the .conf file in that directory. There should be two there, one with the cloud save settings, and the other for all the other DOSBox settings.