I want something with a large enough keyboard to actually type on. Which means I'd be happy with a "phone" the size of a paperback.
Have you considered one of the PDA-type phones, such as the XV6800 (IIRC) sold by Verizon? Runs WinMobile 6 or whatever the latest version is, lets you do the Word/Excel thing (even PowerPoint, though that might just be read-only), has a slide-out landscape-mode keyboard, and while it's large for a phone, it's rather less than paperback-sized with a keyboard that's small but usable, unless you have gorilla fingers. A friend of mine has one, and likes it a lot.
I'm considering the HTC Touch Pro once my current phone is eligible for upgrade from Verizon, so I can go converged and not carry a phone AND a PDA. I'd considered it earlier, with the XV6800 in fact, but I got spoiled by the VGA screen on my current Dell Axim xv51 (disclosure - my PDA is a recreational tool, not a business device - I read books, watch video, write notes and Word docs, etc., on it). I could wish the Touch Pro had a CF slot in addition to SD the way the Dell does, but that would make it considerably larger, and the Touch Pro currently takes up to a 16GB SDHC card, so capacity is not likely to be a major issue.