This module allows you to specify a maximum amount of characters that a text attribute can have. One example is if you are selling jerseys and want to limit the name on the back to 15 characters and the number to 2.
Also has support for defining that an attribute must be numeric.
The size of the attribute textfield will default to size="(max chars + 1)" but can also be configured to be set to a certain amount of pixels * max chars.
This will probably not be ported to D6, because I believe Validations API can add this functionality, albeit with a bit more overhead and complexity in config.