I'm sure the cross wasn't angled at 45° - I hope - it needs to be an angle equal to the latitude.
About large sundials, Cassini (the astronomer) build a meridian in the cathedral of Bologna which is 67 metres long. A hole in the ceiling 27m above projects the sun disc on it at midday - this kind of sundial isn't used to tell the hour, but to measure the length of the solar year, and will tell the day of the year (including equinoxes and solstices).