Betelgeuse is almost a million times the size of our sun.
If you haven't ever done so, I strobngly recommend you download Celestia (free, with Linux, Mac and Win versions available). It is extraordinarily cool to see what Betelgeuse looks like from various distances. For instance, if you go to Pluto, and look at the Sun and note how many AU Pluto is from the Sun. (this varies significantly during its orbit) Then go to Betelgeuse and see what it looks like in the sky from the same distance. Zooming in to Jupiter's orbit yeilds surprising results, and I think it give a pretty good feel for the size of that monster star.
Celestia is the coolest program on the planet IMO.
I have it on my home PC. Haven’t touched it in a while. I’ll have to play with it when I get home next week.