Canvas

The -Paint script block runs while the deck is built, not once per frame. It receives the canvas as $_, with $Width and $Height available for drawing.

New-Slide 'Diagonal' {
    New-SlideCanvas -Width 16 -Height 16 -Paint {
        $_.Fill('grey15')
        for ($i = 0; $i -lt $Width; $i++) {
            $_.SetPixel($i, $i, 'lime')
        }
    }
}

Use -MaxWidth to scale a large canvas down to the available space.