6" is pretty much standard for e-ink ereaders, although there are some larger ones here:
https://wiki.mobileread.com/wiki/E-bo...#Large_Devices
Any ereader can be plugged into the computer to show up as a removable drive. You can copy books in, but it would be even easier to use
http://calibre-ebook.com to send books.
All ereaders are bookstore-agnostic, and calibre can also easily convert between source formats -- automatically. If a book isn't compatible with your ereader, it will be auto-converted on sending. But you may need to google "apprentice alf" for help removing DRM. (DRM is not bookstore-agnostic.

)
Things to keep in mind:
- e-ink ereaders are great for outdoor use, and long battery life. But they aren't really good at PDF. For PDF, it is best to use a tablet (expensive
) which usually stink at both. You can try k2pdfopt to make PDF easier on e-ink ereaders, though.
- Do NOT get a Nook! They restrict the storage space to Nook-bought books, and only allow a small portion of the space to use for sideloaded books. They are the only ereader that does that.