AU is NOT used to measure size. It is used to measure distance. 1 AU = the distance of the Earth from the sun (93 million miles). This is not size. It is a distance. It's the same concept why we use different measuring standards. If measuring the length of you table, you use inches. If measuring how far you throw a football, you use yards. If measuring how far you drive, you use miles. AU is just a measurement standard. It makes taking about large distances easier without the numbers being too large to be hard to use and conceptualize. If I drove from one city to another, which is 320 miles away, what if I gave you that distance in inches? It's a huge number and difficult to deal with. After AU, the standard we use is light years.