The formula that you need is, Radius = TAS2 divided by (g x Tan bank angle). The conversion of kts to m/s is kts x 0.515

 

Therefore 500 x 0.515 = 257.52 = 66306

 

g = 9.81m/s squared or 10 for all practical purposes.

 

Tan30 = 0.5773502 x 10 = 5.773502

 

R = 66306 divided by 5.773502 = 11485m

 

Therefore diameter = 2 x 11485 = 22970m

 

Answer approx 23 km.