How to create dialog box that's properly scaled and appears at bottom of screen?

Hello,

I'm working on a simple 2d rpg game and I'm having trouble figuring out exactly how to use the UI tools to create a dialogue box that appears every time I want a character to say a line of dialogue.

I want the dialogue box to appear at the bottom of the screen and be properly scaled to the camera's aspect ratio and field of view.

I've tried fiddling with the canvas by resizing it and even creating a second camera that only shows the layer containing the canvas, but I can't quite get the behavior that I want. I watched the UI tutorials but I'm still not exactly sure which render mode I want, how to keep it aligned to the bottom of the screen, and how to keep it scaled.