hello. i'm really sorry, i'm very sick with a virus right now and can't get to my pc. so if you don't mind, i'll try to answer your questions step by step in text for now.
- - - - 1. about Palette Generator:
the main feature here is that it works like a "gradient map" in photoshop or clip studio paint, but specifically for your brushes.
let's say you have a multi-colored brush (you can use the built-in ones as an example). if you enable "Gradient Map" in the plugin settings, your brush will be dynamically repainted using the colors from the generator.
to get those colors, you first need a base. click "Get From Palette" (takes colors from your standard aseprite palette) or "Get Brush Colors" (extracts colors from the active brush). the plugin will generate mathematical variations in the rows below (Shade, Saturation, Hue).
so you can use it to instantly remap your multi-colored brushes on the fly, or you can just use it as a convenient color picker for your art - left-click a generated square to set your Foreground color, or right-click for Background color.
- - - - 2. about live preview for brush size:
the thing is, aseprite only shows the brush outline when your mouse cursor is actually hovering over the canvas. when you click buttons or drag sliders inside the plugin window, your mouse is outside the canvas, so aseprite hides the preview.
to see the size changing live, just move your cursor over the canvas and use standard aseprite hotkeys to change brush size (usually "+" and "-" keys on your keyboard, or holding alt + mouse drag). the custom brush will scale and you will see the preview updating in real time.
if this is still a bit confusing, please let me know. i will record some short video gifs to show how it works as soon as my fever drops and i can sit at my computer...