Canvas changes size between sessions (because of two monitors?) Help?

[FltDirErin]

Hi, I'm a newbie and I don't know the terms for things, so I'm struggling to find what to search. Hope you can help!
I am using OBS as a virtual camera with a green screen.
I have two computer monitors: My smaller laptop screen, which sits in front of a larger monitor to make one big desktop. I didn't think this was relevant, but...
Sometimes when I open OBS, a scene I've made full screen in a previous session only takes up a quarter of the canvas... and sometimes in the same session, another scene that I'd previously made is so big only a quarter of it can be seen. I'm guessing it has to do with the monitors, but I don't know how to fix it. Is there a way to lock it in so it doesn't changes after closing/reopening? And is there a way to resize the whole display, rather than resizing the individual elements?
Thanks for any/all help!

 

[FerretBomb]

It shouldn't ever auto-resize, if you're using OBS Studio (bets are off if you're using hacked-up derivatives like SLOBS).
If you want to intentionally change your base canvas size and store it, the Base Canvas (and Output Resolution) is stored in the Profile (at the top of the OBS window). The sources layout however is stored in the Scene Collection (also at the top of the OBS window).

I maintain a 720p Profile and Scene Collection, and a 1080p of each. When swapping Profiles, the scene WILL look jumbled and messed-up until I also switch the Scene Collection to the right one.

Alternately, make sure your Preview window isn't set to 'Canvas'. Right-click the Preview window, go to Preview Scaling, and make sure it's set on 'Scale to Window'. Canvas-scale is good for making fine adjustments without needing a fullscreen preview active, but it CAN cause confusion since it tends to crop the canvas down to show it true-size.