Question

What is the diameter of the universe?

Answer 1

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.

Explanation:

Universe as `the` universe is 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