Answering my own question, seems like a pretty large aircraft. From Wikipedia and a nice picture os a potential layout for pax.
The A330 MRTT has a maximum fuel capacity of 111,000 kg (245,000 lb) without the use of additional fuel tanks, leaving space for 45,000 kg (99,000 lb) of additional cargo.
The A330 MRTT cabin can be modified to carry up to 380 passengers in a single-class configuration, allowing a complete range of configurations from maximised troop transport to complex customisation suitable for VIP and guest missions. Available configurations include 300 passengers in a single class and 266 passengers in two classes. The A330 MRTT can also be configured to perform Medical Evacuation (Medevac) missions; up to 130 standard stretchers can be carried. The main deck cargo configuration allows carriage of standard commercial containers and pallets, military, ISO and NATO pallets (including seats) and containers, and military equipment and other large items which are loaded through a cargo door. Like the A330-200, the A330 MRTT includes two lower deck cargo compartments (forward and aft) and a bulk area capability. The cargo hold has been modified to be able to transport up to eight military pallets in addition to civilian unit load devices (ULDs).
An optional crew rest compartment (CRC) can be installed in the forward cabin, accommodating a spare crew to increase time available for a mission. The passenger cabin of the A330 MRTT can be provided with a set of removable airstairs to enable embarkation and disembarkation when jet bridges or ground support equipment are not available.[
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