I don't do Youtube videos, so this is the next best place I can think of to write these notes down. This thing especially has bothered me for a while because I can't find answers online anywhere. That's probably because it's such a non-problem for every Blender user so nobody talks about it! But I am so glad to finally know this, I need to write it down, just in case any other Blenderists out there who have been just as confused as I have been are looking for this information.
How on Earth do I resize this border thing in Camera view (Numpad 0 - not top of keyboard horizontal 0, Numpad 0!) so it can fill up the view port more, and I can see better what I'm setting up for render?
You uncheck this box that you probably learn to tick immediately from watching Blender tutorials, like me. Un-tick this box by pressing "N", click View tab, and un-tick "Lock Camera to View":
Then, middle mouse button scroll wheel up and down! And then, re-tick that "Lock Camera to View" check box again so you can go back to setting up the view you want, but this time in a bigger box and you can see better!
This seems like such a simple, simple, obvious thing that nobody should ever need to even ask, but it sure has confounded me for weeks! I googled to no avail, asked around to no avail - nobody seems to know what I'm talking about! I don't even know what that box is called! And now I finally figured it out I can't contain my excitement. Even if this helps just 1 other confused Blenderist like me out there, I'll be happy they probably saved themselves weeks of squinting at their camera view while confused and annoyed.