In my area (West Central Missouri), watershed to pond surface acreage ratio rule of thumb seems to be 10 to 1. That's 10 acres of watershed for every 1 surface acre of pond water. Make sure you consider the local ratio for your new pond build.
And, if money allows...build a sediment pond up stream, so to speak, from your pond to catch debris and run-off soils. This pond could also harbor FHM's and act as a forage pond as well. I wish I had one.
In my opinion, islands are not so great. They look cool and lend themselves to a unique pond, but they aid the "filling in" of the pond over time. I would rather have a deep pond all the way across than the aesthetics of an island that allows more vegetation and speeds up the eutrophication process.
