@buzzfever, I haven't tried it, but it should be fairly straightforward. Open includes/languages/english/create_account.php and copy/paste one of the other fields as an example, then replace the variables with your 'extra field'

@gunni, CSS should help you with the length of the form field. Try adding something along these lines:

input {
length: 150px;
}

If you're talking about the actual number of characters in the form field, you could fix this limit in MySQL by going to customers table, and under customers_extrafield change varchar to a larger number.