Can the SSD be upgraded in size? If yes, what form factor is it?

Yes, the SSD hard drive that it comes with, a M.2 NVMe PCIe 2280, is upgradeable. It also has an unused bay for a secondary 2.5 inch laptop hard drive.

Will this run Windows 11?

YES it can. I purchased this and installed a 1 T SSD in it. I kept the M.2 as my C:/drive and made the new SSD a D:/ drive. The operating system and all my programs go on the C:/ drive, and all the program files go on the D:/ drive. Works like a charm.