If we parked Betelgeuse just outside the Solar System, how big would it look from Earth?
The distance to Betelegeuse is not precisely known for reasons you can read about here and here. But let's assume a likely distance of 200 pc. The angular diameter of the star has been measured with optical and IR interferometry to be about 0.05 arcseconds (see the relevant section of the wikipedia page on Betelgeuse), though this is uncertain by about 10%.
These two numbers translate into a linear photospheric diameter for the star of 10 astronomical units (au).
We then have to interpret what you mean by just outside the Solar System. If you mean half way to the next star - i.e. around 0.6 pc, then a star with this diameter would have an angular diameter of 6 arcseconds - so similar in size to Mars viewed from Earth. However, if you meant just beyond the edge of the Kuiper belt at say 100 au from the Sun, the angular diameter would be about 6 degrees! This is more than ten times the size of our Sun, which would indeed look very impressive if it were at all possible for us to view such an event.