Yeah, if price is an issue, I'd probably pass on the Sony too. Their Readers do have a pretty good interface, dictionary and notetaking abilities, etc., but it's not worth the premium if you're not interested in those features.
Nook is a 6". From what I understand, the advantages: tightly integrated to the B&N store, the touchscreen at the bottom. Disadvantages: shorter battery life than other e-ink ereaders, books purchased from B&N use a DRM unique to that company (probably strippable). Lots of Nook fans here though, I'm sure one will come along soon enough to answer your Qs.
Kobo has deals with various companies in various countries. In the USA it's Borders that sells the devices for them, yes.
Sorry about that. I was meaning to say the physical shape and weight, the ergonomics. See what you're comfortable with. For example, the Kobo is pretty light, but you'd probably hate it if you're a lefty. For some people, the Nook takes some getting used to in swiping pages. The 350 feels
too light for some people. Etc.
All of them can change text sizes, yes, at least five levels. One of the things I love best about these ereaders.