I’m working on a 27" monitor so have quite a bit of screen real estate to play with. Of course when I’m viewing my page in the editor at the default 1200px it is really narrow to work with. If I change the width to say something like 2000px it switches to Custom width, and then the preview is not full width but left-aligned.
The width setting you’re referring to is the width of the page you’re designing, so just keep that in mind. Someone on a 15 inch laptop will have a lot of horizontal scrolling when they load a 2000 px page in their browser.
Try leaving the page width at 1200 and then making your page responsive (uncheck "This page is fixed width) so that the run mode on a larger screen, Bubble’s responsive engine will allow the elements to stretch out. You can adjust the behavior in the Responsive tab of your editor (give things a max width, etc.).
It sounds like you already have it in responsive mode and your element’s margin alignment are all left-aligned. You can change this to center-aligned in that Responsive tab. The outermost parent element (I assume a group contains everything?) would need to be centered horizontally to remain centered no matter the screen width.
Note that you can still design an app with a 2000 px setting and make that responsive so that a smaller screen squishes things down, but I find it a LOT easier to design smaller and allow responsive to handle wider screens than the other way around.
As far as your view of the editor, you’ll probably just need to zoom in on your browser to get a more comfortable work space. Would be nice to have a Bubble-native zooming tool for the editor though…