A user recently inquired (OK, complained) as to why a 19-digit account number on our web site was broken up into 4 individual text boxes of length [5,5,5,4]. Not being the original designer, I couldn't answer the question, but I'd always it assumed that it was done in order to preserve data quality and possibly to provide a better user experience also.
Other more generic examples include Phone with Area Code (10 consecutive digits versus [3,3,4]) and of course SSN (9 digits versus [3,2,4])
It got me wondering whether there are any known standards out there on the topic? When do you split up your ID#? Specifically with regards to user experience and minimizing data entry errors.
I know there was some research into this, the most I can find at the moment is the Wikipedia article on Short-term memory, specifically chunking. There's also The Magical Number Seven, Plus or Minus Two.
When I'm providing ID's to end users I, personally like to break it up into blocks of 5 which appears to be the same convention the original designer of your system used. I've got no logical reason that I can give you for having picked this number other than it "feels right". Short of being able to spend a lot of money on carrying out a study, "gut instinct" and following contentions from other systems is probably the way to go.
That said, if you can make the UI more usable to the user by:
OR
"_____ - _____ - _____ - ____" and ends up looking like "1235 - 54321 - 12345 - 1234"It would almost certainly make them happier!