The opposite side of this island has cabinets, so its filled in. The dimensions of this island are 7'-6"x3'-6".

This is a good size island. I am debating putting in a prep sink in an island of similar size, just slightly wider. Would there be enough area to prep and entertain with something like that or would I be better off leaving it as a big work surface and extra storage?

Thanks for your question Shellabella. In our opinion it's always nice to have a prep sink in the island. If you are worried about enough working space just keep it small, you don't need a huge sink.

Whats the dimension between island and countertop

Thanks doe your question nojov. The dimension is roughly 48".