= Light-crossing size bound for a variable source
{title2=$L\lesssim c\Delta t$}
If a single approximately stationary emitting region changes coherently through a causal disturbance, a disturbance travelling no faster than the <speed of light> takes at least $L/c$ to coordinate a scale $L$. Light-travel differences also smear simultaneous emission from different depths. Under those assumptions, observed intrinsic variability on a time scale $\Delta t$ suggests $L\lesssim c\Delta t$.
For a chosen example $\Delta t=0.010\,\mathrm{s}$, multiplication by $c=299{,}792.458\,\mathrm{km\,s^{-1}}$ gives $L\lesssim2998\,\mathrm{km}$. This is an order-of-magnitude <causality> bound on a coordinated emitting scale. It is not a measured stellar radius, and even the relation between $L$ and a diameter depends on geometry. A <pulsar> beam crossing is a geometric clock, and <pulsar dispersion delay>, scattering and instrumental response can broaden its observed pulse. The recurrence <period>, observed pulse width and intrinsic variability time must therefore be distinguished before applying the bound. Relativistically moving emitters require further arrival-time corrections.
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