Assuming that particle consists of a photon that moves circularly in the loop, creating standing wave obeying the rule that its closed path 2*Pi*R=n*Lambda (1) (resonator where resonator length 2*Pi*R is „n” times photon wavelength) and knowing particle's mass (experimental value), we can calculate its radius.
Lets put c/v (2) for Lambda in the above equation (1), where v is photon frequency. We get then 2*Pi*R=n*c/v (3). Now if we assume that particle's mass is of EM origin and m=E/c^2 (4) where E=hv (5) is the energy of circulating photon as described above we can rewrite (4) as m=hv/c^2 (6) or v=mc^2/h (7) (letter h stands for Planck constant of course). Now lets put (7) into (3) and we get 2*Pi*R=n*c/(mc^2/h) (8) or simplifying R=n* h/2*Pi*mc (9).
Now, let's take proton for our considerations. Assuming n=4 in eq. (9) and m=1,672621637(83)*10^(-27)kg (experimental value) we can calculate proton's radius to be R=0.84124 fm which stays in agreement with the experimental value of 0.84184 fm +/- 0,00067 fm (the most accurate experimental value measured in a Hydrogen atom with a Muon in 2010).
You can read more about that theory and mechanism in my paper here:
https://www.researchgate.net/publication/333732258_GENERAL_not_grand_UNIFICATION_THEORY_Quantum_Gravity_and_Schwarzschild_Particles
What do You think?