Data Matrix 2d barcode expert needed

We are exploring the idea of using the Data Matrix 2d barcode. We need an expert on Data Matrix 2d codes to answer the following questions: * What is the smallest printed size of a printed Data Matrix code that: (a) can be read by an smart phone; and (b) will contain a URL of up to 100 characters? * What open source libraries are available for generating these codes? Also, please provide a PDF with an example of the smallest Data Matrix code that links to a 100 character URL. We will test this with both an Android phone and an iPhone. We expect to be able to scan the code with an app such as Barcode Scanner 3.5, available free on the Android Market.