Facebook profile/fan page image: Up to 540 pixels long by 180 pixels wide
Front page box under photo: 242 characters to write a brief description – include keywords whenever possible.
Status update posts: 420 characters. You can sometimes get more if you are copy/pasting from another program. You also get apparently an unlimited number of characters for Facebook Events.
On the Info Tab:
Website: 720 characters. You surely don’t need that many but if you have several websites, include them one per line, being sure to include the http:// part. This section turns every word into a URL so only include website addresses.
Company Overview: 840 characters to describe what your business does, who you serve, how you serve customers, where you are located, etc. Include anything you want your readers to know about your business and how they can do business with you.
Company Mission: 840 characters to describe your company mission.
Products: 600 characters to describe the types of products you offer your customers, if any.
Custom background image: 1255 x 555 pixels max, with the central space for the twitter feed being 450 pixels wide. Arrange your image so that anything you want to show is separated by 450 pixels of blank space.
Bio: 160 characters. Abbreviations are Ok!
Posts: 140 characters. Abbreviations are Ok!
Posts: No limit, however some users have had errors over 1,000 words. For the reader’s sake
as well, it is a good idea to break the post up either by inserting the “more” tag (option-shift-T on
a Mac, or alt-shift-T on a PC) which tells the reader to click to read more, or the “page break” tag (option-shift-P on a Mac, or alt-shift-P on a PC) which tells the reader to go to the next page. These are also buttons in your post editor – they look like 2 blocks with the dotted line in between them.
Header Image: These vary according to the theme you are using, but in general a good size is 760 pixels wide by 200 pixels high.
The following vary in what is shown and used in the different search engines, but are on the lower end so they should work for each one:
Title: 65 characters
Meta Description: 150 characters
Meta Keywords: 256 characters