Let the minimum frame size is F .
F ≥ 2 × bandwidth × td ....(i)
where td = propagation delay = distance / link speed
now putting value into (i) we get F ≥ 2 x 512 x 106 x ( 2 / 200, 000 )
F ≥ 1024000000 * 2 / 200, 000
F ≥ 2048000000 / 200, 000
F ≥ 10240 bits
8 bits = 1 Byte
10240 bits = (10240 / 8 ) bits = 1280 Bytes