Information usually contained in a sig block includes the poster's name and email address, along with other contact details if required; URLs for sites owned or favoured by the author; and a witty or profound quotation is often included, or an ASCII art picture.
Since by definition these blocks are added automatically to a message, usually regardless of its content, there are guidelines of netiquette regarding their size. The most common guideline is: no more than four lines of less than eighty columns each. This keeps the overall size of the message down, conserving bandwidth as well as the time required to read the message, and ensures that eighty-column terminals (the most common terminal width by far) can display the sig block properly (sometimes the last column is reserved for displaying a continuation character, so using all eighty columns for text can result in a character wrapping to the next line).
The formatting of the sig block is prescribed somewhat more firmly: it should be displayed as plain text in a fixed-width font (no HTML, images, or other rich text), and must be delimited from the body of the message by a single line consisting of exactly two hyphens, followed by a space, followed by the end of line. This latter prescription, called "sig dashes", is both easy to remember, and allows software to automatically mark or remove the sig block, as the receiver desires.
However, whether due to ignorance or disregard for these guidelines, a great many people use sig blocks larger than these dimensions, or formatted improperly. In past decades, such practice was referred to as warlording, named for one particular Usenet regular who openly flouted the guidelines with sig blocks stretching to many hundreds of lines of ASCII art on messages with little or no relevant content.
Many corporations have internal policies requiring outgoing emails to have lengthy "signatures" appended to them, listing dozens of contact methods, disclaiming legal liabilities, notifying of virus scanning and so forth. These corporate signatures are almost universally large (often larger than the message itself), and composed without regard for the netiquette guidelines described above; they are seen as obnoxious and irritating by many who receive them.