Here goes, I have a feeling this is totally and utterly wrong, oh the shame!
1 light year = 9.4605284 x 10^18mm
Aldebaran = 65.1 million light years away
65.1 x 9.4605284 x 10^18
= 6.159 x 10^11
Diametre of Aldebaran is approx. 61 million km
19.41/6.1 x 10^12mm
= 3.18 x 10^12
6.159 x 10^11 x 3.18 x 10^12
= 1.958 x 10^10mm
= 1.958 x 10^4 km
= 19580km
