The length of the song is determined by the song structure and the different length and count parameters.
If you want to make the songs longer you can take a look at the song structure for the "Sentimental" song template, which uses some more parts (and also probabilities for them to exist at all within a song). Then you can also change the various "Length and Counts" to modify the number of possible phrases within each part and also the number of measures for those phrases.
I know that this is rather tedious, and I wish that there was a more intuitive option available for controlling the song length.