I thought it would be helpful if I shared the character count limits and image size recommendations for the various social media sites. If you know what your limits are ahead of time, you can create something offline to maximize the space and then insert it into your profiles. I based these on my category of professional services. I am not sure what the limits are for other types of businesses. All character counts below include spaces. Of course you want to use good keywords whenever possible, to attract people through the search engines.
Updated April 2016 to reflect Facebook changes
Facebook Business Page:
Facebook profile image: 180 x 180 pixels
Cover image/banner: 828 x 315
Image in an Image Post: 1200 x 630
Image in a Link Share Post: 1200 x 630
Status update posts: The ideal length is 40 characters, however you are technically able to use 63,206 characters.
On the Info Tab:
Founded: 255 Characters
About: 255 Characters, 60 of which show on the left sidebar of the Page
Description, Products, Awards, Website (one per line, full URLs), Biography, Personal Information, Personal Interests and others: Unlimited
Facebook Event Descriptions: Unlimited
Custom header image: 1500 x 500 pixels
Profile Image: 400 x 400 pixels
In-stream Image with Post: 440 x 220
Bio: 160 characters. Abbreviations are Ok!
Posts: 140 characters. Usually best to do 125-130 so there’s room for your username for retweets.
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