3,5" is a form factor. PCIe has plenty of connectors and cables and drives will definitely stay 2,5"/3,5".
Not necessarily! When drives got smaller historically, system builders took advantage of that to build smaller, lighter systems. This was the history when drive manufacturers introduced hard drives of 8" size, then 5.25", and so forth. Certainly with SSDs or even HDDs, why not a 1.8" form factor, once the capacity and price are where the OEMs need them to be?
Check this out. https://www.google.com/search?q=1.8...drive&rls=org.mozilla:en-US:official&tbm=shop