What does this number measure?
Order of magnitude: about a thousand solar diameters according to this ESA presentation. The inferred size depends on wavelength, adopted distance and the star's variations.
Compare the stated dimensions
The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.
Source and limitations
Catalogue value: a source is provided, but the measurement has not been individually audited in this project.
Betelgeuse: interpretation of the variable photosphere and large convective cells; approximate boundaries.