Most of the safes are quite small. Big enough to fit a wallet, mobile and passport in. Some cabins have larger safes that can fit quite a bit more in (would suggest not big enough for most laptops however).
I have been on the Oceana three times. Each time we have had a JB grade balcony cabin. The safes are large enough to put a small camcorder and a camera in along with passports, money and other bits and pieces.They are about 11" wide 7" high and 7" deep. With regards to free upgrades on P and O, forget it they don't happen,well not in my experience anyway. The ships are nearly always sold out for each cruise.Hope you enjoy your cruis.