- My email tiles are the smallest (1x) size because they still show unseen count at that size (also shown on the lock screen), which is the only "live" information I care about for them. At 4x they will show the name of the email account (which I don't need, since I can tell by the icon) and at 8x they show the subject line of the most recently received email, which I don't personally care for.

- Same goes for my phone and messaging tiles, as well as IE and maps, which have no live functionality

- I use a 4x tile for my calendar for two reasons: 1) I've put it in the top corner of the screen, and 4x makes it easier to reach if I want to open the calendar app, and 2) at 4x it shows the day, date and next calendar appointment (also shown on the lock screen, but I like it here as well).

- The pinned-contact tile for my wife is 4x because the vast majority of communication I do on the phone is with her, and because I like seeing her picture there. Plus, at 4x, it shows Facebook status updates and sometimes other stuff.

- I keep the Weather Channel tile at 4x because it shows today's forecast.

- Pinned websites are all 1x, because they don't update and I can tell from even small icons what they are.

- A few other apps that I use (I personally don't use a lot of apps except the stock ones) are pinned at 1x, since they don't have any live functionality I really care for.

I've always thought the "live tile" idea was a bit oversold since in the majority of cases I don't really find it useful (I don't really count things like email unseen counts as live tiles - even if WP8 didn't have live tiles I would still expect that information to be there, like it is in iOS), but the new WP8 ads about being able to really make it "your" phone definitely ring true. If I had other apps, workflows or priorities it would look completely different.

disclosure: works for MSFT.