I'd really need your island in relation to the rest of your kitchen, particulary the appliance placement. It's wonderful you have so much space!

If you have proper clearance between your layout it should look fine, I like a big square island especially with raised panels and maybe some corner post then bump out the counter to match. You just want to make sure you have enough room in between the island and appliances even with them open, so consider things like do you have a single door frig or french door style. also dw door and oven door clearance you don't want to be pinned to the island when you look into your stove 42" clearance is a good number if you can get it.

Hard to tell without seeing your layout and also knowing what style island you want...But mine is 48" x 60" with an overhang on one end. It's squarish, but I wanted it that way and it goes with the style I was after.

I think it's more important to have proper clearance around your island than to determine which shape is better (more rectangle vs. square). With 42" you have enough clearance, so I would go with what you prefer instead of your cabinet maker (and no, I don't think it would be too wide at all).

My island is 6' x 14' with 48" clearance and it is proportionate to the size of our kitchen. The island size was determined last; it was what filled the space perfectly.

I don't have an island, so I can't speak from experience, but I have a few questions for you. How do you plan to use the island? Will there be anything on it like a cleanup or prep sink, or cooktop? How tall are you and other kitchen users?

Who will sit at that long overhang...e.g. do you have small kids who will (now or someday) need help w/ food/drinks/homework while you work? In that case, the narrower island might be better so you can reach their stuff...could you reach the opposite side of the 48" island to refill a drink, quickly look at a math problem, etc? Or would you have to go around? Would they be able to reach something they wanted to help with? (These questions could apply to adult visitors, too, not just kids).

Would you be able to wipe the counter easily from the "cook's" side, or would you have to circle the island whenever you wanted to wipe up...and if so, would that bother you?

Socially, would you want someone seated at the overhang to be 4' away from you? Or would that be a good thing, if you have a sink which might be splashy, or a cooktop that might be spattery?

Just a few things to think about. Having the 48" top means you could have a few 12" deep cabs on the seating side...in my book, that would be great, plus all that lovely acreage for baking projects, etc. But be sure to think about how you see yourselves living/working in that kitchen.

I would make a cardboard or table mock up in the size you want and the size the carpenter is recommending and spend a day trying to work around it.

Horking big islands are the trend now, but can be a real pain to work around in a kitchen. See if you can live with one that large. Also agree with posters above - you want it to fit into your kitchen in terms of walkways and just generally be in proportion to the rest of the kitchen.

Just one more thing to think about is ease of wiping down the counter surface. About 36" is recommended if you want to be able to reach over and wipe the whole surface. May not seem like a big deal, but can turn into a pain on a daily basis.

42" between countertops is considered a minimum aisle.

A 48" wide island often becomes somewhere around 49.5" to 51" or so by the time the doors and countertop overhangs are added. Base cabinets up against walls often have some depth variance to wavy walls, so your actual countertop edge may be out as much as 26" from the wall (or maybe not or maybe more) normal countertop depth is around 25" to 25.5" if your walls are perfect.

A 42" aisle BETWEEN CABINETS often becomes 39" between counter edges. A 39" aisle would drive me insane, but you might be ok with it.

So, something to ask about before committing.

Agree on needing the 42 inch clearance. Also, why not do a paper cut out of the size, but it on a few boxes and pretend you're cooking, cleaning, etc. Make sure you can comfortably reach across the bulk of the space without straining. Also, are you putting a sink on the island --I'd highly recommend that.