Agreed that it will be tight for adults. 24"-30" is the recommended width, with at least 15" of overhang. If you don't have the depth of overhang, you'll have people sitting sideways, and it for sure won't be long enough. Look at narrowing up the cabinets on the island to be able to do an overhang on one end. And don't forget to deal with the counter support issue now, in the design phase.
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