A "maxlength" parameter is currently already provided, however I have heaps of use cases where a minimum length would come in handy too, e.g. all sorts of IDs like ISSNs, ISBNs or postal codes, phone numbers etc. I am not sure if a fixed length could be done with just a combination of min and max or if a fixed will be better for this case.
Yes, though it is not yet a frequent thing. Currently I use the #len parser function to check the length and provide a warning with other functions after the page was saved. I do this for IBANs.