Want something that folds perfectly to A4? A3. One more fold? A2.

This made it sound like you meant that folding an A3 sheet twice gives A2 😅 Might just be my reading comprehension, though!

Cs are the envelope sizes.

That's really weird.

There is also B4, which is 353 x 250 mm. Not perfect, but you can more easily see how they got 250, as B0 is 1414 x 1000 mm.

There is, but A4 isn't the baseline, A0 is.

A0 is 1m² with a ratio of 1:√2. Cut that in half along the larger side and you have A1, half it again and you have A2, etc. on down the line.

The entire series maintains the same exact ratio down to A13 at 9mmx13mm.

I see what you’re saying, but the «weird» dimensions are what makes the whole thing with folding in half to get the next size work:

The aspect ratio has to be 1 to the root of 2 for it to work, and that’s going to make some dimensions «uneven». The utility of that system is worth it many times over!

It's based on the 1m² A0, fold it in half to get A1 and so on.

The proportions stays the same too