After over an hour of math, I think I have come to the conclusion that an Ursa Major would be about 1683.03435 times bigger than Twilight Sparkle. So really about 1683 times larger. Twilight is around 58 pixels tall on my screen and the Ursa Minor next to her is334 pixels tall which after a long equation makes the Ursa Minor 5.7586205895... times larger than Twilight. The Ursa Major is 5.39025 times larger than the Ursa Minor. So then I took 58 times 5.7586205895..., which is the height of the Ursa Minor 334 so take that times 5.39025 and you get 1683.03435 which is approxamately how many times larger (In height) an Ursa Major is than Twilight Sparkle.
Oh yeah, and I forgot to mention, I don't own a calculator so all of this was done by hand.
@majorly awesome I hope this is what you were looking for because I wasted a long time trying to figure that out. Well I guess it wasn't a total waste considering I know how large an Ursa Major is compared to Twilight