Keep in mind that those are general design choices, and that the purpose of a passepartout is to make your artwork stand out: if that means the borders of your passepartout are extremely small or large, or the upper border is broader than the lower one, or the left border is missing, that is completely fine.
As with all things creative, it's your choice.
Where can you recommend to read about such design considerations?
I don't think the question should be 'where to read about', as recommending places seems off-topic (e.g. "the library!", or "google!").
I think 'what other considerations should I take into account concerning design choices' is more appropriate.
- Something I have learned was to cut the inside edges in an angle, making the hole in the passepartout increasingly larger from the artwork towards the glass panel/observer.
Depending on the material used, this could mean a subtle change in colour will be visible (as the inside of the material could be less bleached or foamy, for example) around the edges, which is something to be mindful about.
- You should consider the shape of the passepartout, as well: some artworks might be accentuated by an oval- or trapezium-shaped cutout.
And, even if the cutout is rectangular shaped, you can change the shape of the outer edges, as well (limited by potential framing, of course).
- You can layer passepartouts. Mix different shapes and colours, use the relief you get this way to create a three-dimensional shape (like a landscape).
- Apart from the surface of the passepartout material, the edges can be coloured, or even painted, too.
In addition to the other options, this allows for an infinite (and possibly overwhelming) range of possibilities and creative solutions.
Some material-oriented choices you might face :
- Make sure the passepartout material is acid-free, to prolong the durability of your artwork.
- Take heed of the thickness of the material. This determines not only the look of your artwork, but is also important when considering framing the entire piece.
- This might be beyond the scope of your question, but, depending on where the framed passepartout is going to be hung, you might consider putting UV-resistant glass in the frame. This will let both your artwork and your passepartout (especially light-coloured ones) retain their colours better.