Are all the quartz slabs going to be too small or just the Frosty Carrina?

What size is your island?

Anything Caesarstone will be too small. They don't make jumbo slabs. I think Silestone (Lagoon?) would work, but I don't know that I love it. I couldn't find a single granite or quartzite that I liked, other than Sea Pearl quartzite but all the samples were very green and I really want a white/gray marble-y look.

What a pain to find that out after the fact! We designed our island to be essentially the length of a Ceasarstone slab.

So put a seam in it and be happy.

Ditto Trebuchet. There is absolutely nothing wrong with having a seam in your island. If your fabricator can't do a good quality seam, then you shouldn't be using them. Period.