Does the radius (shown below) take into account smaller objects (FOD), or only people? How is it determined?
The calculation is based on experimental data and research on better understanding the role of the vortex action. For example:
Scaling laws are derived to relate model and full-scale tests end experimental evidence is cited to demonstrate their validity. For a given inlet velocity the strength of the vortex, and thus the maximum size of particle that can be lifted, is very much a function of the ambient vorticity, end of the strength of the wind blowing on to the intake [...]
— Glenny, D. E., and N. G. T. E. Pyestock. "Ingestion of debris into intakes by vortex action." (1970).
Manufacturer data also comes into play. The F-16 in your photo – based on a US military manual I stumbled on – its ground marking is optional, and the manual doesn't state why the F-16's intake hazard is unique to warrant a marking while the other jets are not mentioned, but based on available literature, it's likely due to its intake diameter to height ratio.
15' [4.6M] IDLE (SHOWN)
25' [7.6M] MIL THRUST (NOT SHOWN)
8-17 F-16 ENGINE INLET DANGER AREA PAVEMENT MARKINGS.
In an effort to reduce the hazard caused by F-16 engine inlet suction, F-16 parking positions are optionally marked in accordance with Figure 8-13; however, aircraft maintenance personnel determine whether the radius is established at 15 feet (4.6 meters) for idle thrust or 25 feet (7.6 meters) for mil-thrust.
"Hazard caused by" here should be referring to hazard to maintenance personnel. Logical practice dictates keeping the area clear of any items as well. The engine manufacturer would have provided the data, similar to civilian aircraft: How long to clear the 'suck zone' of a turbofan after start is initiated?