You can't. That's all there is. There are 3.5", 2.5" and M.2 (Both SATA and PCI). That's it. There is no 2" or 1.5". 2.5" is the smallest non-M.2 consumer drive size on the market. There are some 2.5" drives that are thinner or thicker, but there are none with a smaller footprint.
That's true, and I don't really think about those because it's a mostly dead technology that never took off and nobody much uses. I would not however buy one off Ebay. Too many instances of the manufacturer not being willing to honor warranties on Ebay purchased electronics due to sellers not being authorized dealers of those electronics. Sometimes they will, sometimes they won't. I wouldn't buy ANY hardware component, especially a storage device or graphics card, without a warranty that is unquestionable.
This is an option then, if you want to look at those, but as mentioned you'd have the additional expense of adapters as well.