I previously wrote how if the QR Code you generate resolves to a url it is a good idea to make sure that the page is optimized for display on a mobile device The first rule of QR Codes.
The second rule of QR Codes is to make sure the url is as short as possible.
The reason for this is that you want the code to be easy to read and to reduce the risk of error when read.
Many mobile devices and reader software have difficulty with a QR Code matrix greater than 33×33 and some even falter with those dimensions. This means you should aim for a small matrix rather than a large matrix.
In QR Code encoding the number of bytes at a given Error Correction Level (ECL) will determine the matrix size (see chart below) therefore a shorter url can mean a dimensionally smaller matrix.
If you are concerned about the persistence of a third party url truncation service you can always register your own short url and perform a redirect. There are still lots of unregistered random five letter/number combinations.