What is the diameter of the universe?
It is better to address universe as our universe or observable universe. Our galaxy Milky Way (MW) diameter is about 3.3 billion AU. The dimensions beyond MW are undefined.
As of now, it is reasonable to confine ourselves to our galaxy Milky Way, for estimating distances.
We are at 25000 - 27000 light years, from the center of Milky Way. So, its 2-significant digits (sd) diameter, in AU, is 2 X 2,6 E+04 X 6.3 E+04 AU = 3.3 billion AU.
1 ly = 62900 AU, nearly.
International astronomical Union (IAU) definition AU = the mathematical average of Sun-Earth distance = semi-minor axis b of the Earth's elliptic orbit (in 10-sd) = 149597670.7 km.
As of now, the precision is getting lesser for stars and galaxies.it is It