Size of pipe will also depend on how deep your diffusers are in the pond. It also depends on the operating pressure capactity of the compressor you have. Rotary vane (low psi) vs rocking piston (high psi). For rotary you want less loss psi back pressure compared to rocking piston which has excess psi to allow for pressure loss. If diffusers are shallow such as 8-12 ft you can safely have more psi loss of 1-2psi compared to if the diffusers are 14-17ft deep. As esshup says PSI loss will be significantly less with 1" ID pipe vs 0.75"ID pipe at 500ft. At 500ft for 0.75"ID, 3.5cfm, and 8psi, psi loss will be 1.6cfm whereas with 1" it will be 0.38psi loss.
See this for calculating air pressure loss.
http://www.engineeringtoolbox.com/pressure-drop-compressed-air-pipes-d_852.html 
