Some time back, around mythtv 0.20 you could select the number of rows and timeslots in the program guide. Since then, it's been hard coded in the schedule-ui.xml file and most limit that value to 7 rows (channels) and 4 time slots. So since then I've been modifying these files to allow more rows and more timeslots. It's pita, but afaik, it's the only way to get more. The mythbuntu theme is 7x4, afaik the most of all the themes, so that's the one I used and have since modified it to 14x6. Here it is.
See later post fro newer version.