If you set the Resolution parameter to 200 PPI or maybe 195 PPI (default is 300 PPI) the quality is still good and you don't need to upgrade to the professional version.
Else you can donate here http://www.andreaplanet.com/andreamosaic/donation/
, with PayPal you will get immediately the professional version.
Set Mosaic size to 6' and add use a square sized image for the main image.
You can leave the other parameters as the default values or play with them, raising the number of tiles, selecting one of the various Patterns, and so on.
Once the mosaic is created you can print it in any shop that allows poster printing of a photo.