Hi Lauren and welcome!
The usual overhang is 1 1/2 inches, but you can do anywhere from there to no overhang at all. The purpose of an overhang is to protect the front of the cabinets from drips and to make for a comfortable distance so you're not right up against your handles and knobs when you work at the counter.
For seating, you want a minimum of 12" at counterheight and 10" for bar height, deeper is better, up to 15-18" at counterheight is acceptable if you want more leg room.
If the area behind the island is just a walkway with no seating, you can go to 36". I would not go less than that.
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