What measurement typically defines M87 as the largest object in the Messier Catalog?

Answer

Actual physical diameter

The text specifies that when defining the 'biggest' Messier object, the measurement that secures M87's title is usually its actual physical diameter, which dwarfs spiral counterparts like M31.

What measurement typically defines M87 as the largest object in the Messier Catalog?
sizeNGCgalaxyastronomyMessierMessier object