Is there a website that suggests suitable font sizes for each scale level in the layout?
I am currently using ArcGIS 9.3 but pretty soon will upgrade to 10.
Just "my experience", but we tend to use Arial, bold at a 10 font size for the things we want to stand out on our maps (often with a 1.5 pt halo if we're using LIDAR or other sat imagery). This font stands out and is far easier to read than the fancier font choices. We usually make our maps for 11X17 sheet size, but not always (in reference to that choice of font size).
For features you want to fade to the background a bit, but still show as "relatively important" on your map, removing the bolding & halo and changing to a 9 font works well. For features of even lesser importance, italicizing the text obscures it a bit more (ie - a river or tributary name). Choosing a softer colour also helps to obsure certain labels.
As for sites for you to reference, I don't have a lot of time, but take a peek HERE. I just did a quick google search. Doing the same from your end may yeild results too (see what you can find). Another thing you can do is search "images of maps" or "pretty maps" online, or just grab a book with pretty maps in it. From this, you can attempt to mimmick the font choices that catch your attention. That's what I used to do before working in a standardised environment. I ended up with some pretty eye-catching labels/pointers.
Another suggestion I have is to call your local Municipality, and ask them if they'd be willing to share their standardized font choices with you. I was able to get this information a time-or-two in the past.