Twice the source mean radius; body diameter only, excluding rings.
The Sun changes the scale again. Beside Earth, its diameter is already difficult to imagine; beside the representative Betelgeuse model, the Sun becomes a tiny point.
Choose a reference you can picture
Make Earth a 2 cm marble and compare it with the Sun. Read the resulting length as a miniature diameter, not the distance from Earth to the Sun. This distinction lets you build a faithful size comparison without accidentally mixing size and separation.
Where does a star end?
A star has no solid surface like a rocky planet. The solar reference uses the mean radius in NASA’s Sun Fact Sheet, doubled to obtain diameter.
Measurement source
Sun Fact Sheet — NASA. Twice the source mean radius; body diameter only, excluding rings.