[HELP] Creating a pixel art game for mobile

I am experimenting with creating a pixel-art mobile game in Godot. I am having trouble with the viewport size regarding the pixel art assets. I've learned that I should set the scaling mode to integer, which adds black bars to the screen. My plan to get rid of the black bars is as follows:

1. Find a way to resize the main viewport dynamically in runtime
2. Calculate, based on the device's size, what size the viewport should be
3. Apply that calculation when appropriate

Before I even try to get a calculation like that going, I want to make sure I can change the viewport's size in the code, which I couldn't figure out myself. I have tried the following:

• Changing the project settings in a _ready function: Does nothing
• Changing the project settings in a _process function: Does nothing
• Setting get_viewport().size in a _ready function: Does nothing
• Setting get_viewport().size in a _process function: Changes the size of the viewport, but without expanding it
• Setting get_window().size in a _ready function: Does nothing

What else can I try?