On Horizon & Zenith, the 2 oldest vessles, the safes at the pursur's office were smaller than a safe deposit box in a bank; about the size of a regular envelope only long. On all other ships to date and maybe by now the 2 referenced above, the safes are in your cabin. They are "smaller" than a bread box, more like a mini microwave. Laptops will not fit. Unfortunately due to job demands, it is sometimes necessary to be in touch. Just lock it in your luggage when out of the room. After the first day, you won't see many people walking down the hall with a suitcase! On the newer ships, there is laptop connections in the suites at least.
You can get a cable lock with a combo or key and lock down the laptop to any pice of terra firma in the cabin. I am a consultant with an international firm and that is ow we secure our laptops both on our sites and at client sites. We have very little theft. Hope this helps!
I saw something the other day about securing laptops. Using the cable idea, now this sounds really dumb, but on actual thought...lock the cable around the toilet/head. There's a piece of "furniture" that's not going anywhere. Just be sure that it's not cleaned with the toilet, too. :-)