In the hud layout screen (/hudlayout or main menu > HUD layout), click on each item to select it, then use Ctrl+Home (on a keyboard) or R3 (on a gamepad) to cycle through the available sizes for that item.

You can also do this with individual windows outside the hud layout screen. For example, out of hud layout (when you're playing normally) open any window (such as your inventory window) and make sure it has focus (click on it with a mouse or press select on a gamepad until it's the window with the gold border) and press Ctrl+Home or R3.