Just reporting this, no reply expected unless there is a current fix.
I have a dual monitor setup with my main monitor at desk level and a secondary monitor above it. The desktop is extended across the monitors such that the second monitor is an upward extension of the main monitor.
When I use either the Modern or Win7 themes and open the start menu, it stretches all the way up into the top monitor. The icons are displayed on the top monitor and there is a big blank space on the main monitor.
Neither theme's start menu appears to be resizable.