Posting a layout of the kitchen would help. It is better to skip having an island than having one that makes the kitchen too crowded, but some are also narrower than others.

I used google and came up with this

Our cabinet guy told us the same thing. We bought what we wanted after the kitchen was finished. We called the granite place and ordered a top for the store bought island. I can't imagine not having it. It is the needed landing space for the refrigerator.

It's small. More of a 'kitchen cart' than a full blown island. But it gets a lot of work and it keeps people from walking into the business area of the kitchen.

This is our finished kitchen before we bought the cart/island. We put it right in the middle and we love it.

It's about this size, but had a cheap top. So we added the matching Cambria and it looks great.

What I'm trying to say is, do what you want. It's your kitchen.

you'll want mostly 42". you can do 36" in a pinch if there is no doors (oven, dishwasher, fridge) opening across it and people trying to walk through.