The size (I believe is the same as the other cabins. You automatically get a week-long Spa Pass to access the T-pool/Steam Rooms/Thermal Suites. Because we weren't able to book a Spa cabin because of the Casino Rate we were using, I had to purchase the Couple's week-long pass for $186. From what I recall, you get slippers in your cabin that you get to take home, and nice bathroom supplies for your use. You should also have robes in your cabin for your use onboard; in other cabins you have to request them from the cabin steward. There is a small elevator on Deck 10 to bring you directly into the members-only area of the spa.
It's a larger pool of seawater that's heated, but not quite as hot as a hot-tub. There is a large area of bars you can recline on, with bubbles coming up from underneath, some hot jets, and also two areas where you can cascade the warm water on your head, shoulders & back. It's full of skylights, so it's very light & airy. We really enjoy relaxing there. Because the entire spa area is 18+, there are no children allowed.
I think they say to book flights leaving no earlier than noon. If you are carrying your own bags off, you can get off earlier than those who leave their bags out to be collected the night before. We drive, so I don't really have good info for you on that. The distance from LAX isn't that far, however traffic is always a crap-shoot. Because of a wreck, it took us 2-1/2 hours to get home to San Diego, rather than the usual 1-1/2 hours.