How do I make sure that my custom circuits are scaled correctly?

Is there a specific pixel count or a copy-paste ruler I can use?

Thanks!

If you want them sized correctly use MC1's tiles to be sized fine.
And what I do for mine is my maps are ALWAYS 1200x1200

That's dependent on your style and what you're using to make it. But personally, I have a minimum canvas size of 200x200 in Pixilart. My road widths are 8-12 pixels unless it's in a more open section. This is scaled up by 6 times, so usually I'll have a 1200x1200 canvas with 48--72 pixels road width. Hopefully this helps

1200x1200 is perfect.