Most people make the mistake of concentrating on what the character's service record is. Have you ever looked at your resume? Is it who you are? What makes a character are the things that don't happen during a duty shift. Spock is a fascinating character because of the struggle between his logical and emotional sides, not because he can type three hundred words a minute. We are all bruised and battered by life; that's how we know we have lived. Your character should have a horrible releationship in their past, or an away mission where they got people killed, or left their last post because they decked the XO. This makes the character feel real and gives us the opportunity for plot lines.
Another thing we do that we should not is make our characters more powerful than they should be. Picard wasn't the strongest person in his crew, or the fastest, or even the smartest. He thought out solutions, though, and relied on his crew to implement them. Super creatures are actually very dull to play. They are even duller to play with. All half-Vulcan/ half-Borg/ half-Betazed/ half-Klingons with implanted phaser rifles in their ears will have a transporter accident when coming aboard.
Length is not an issue as far as bios go. Each character will have their own page, so feel free to go wild. If you have a picture you want to use, a link to your home page, or even music, feel free to send it to us. Just make sure you follow any copyright laws and be realistic; not everyone's character can look like Brad Pitt. Only mine can. ;-)