For continuous stills in RAW @ 3.9 fps, 100MBs write speed.
Video requires a lot less, like 6 MBs.
I use 80 MBs cards, but I rarely shoot rapid fire.
I know the question is regarding size but since the speed was mentioned, just remember, your 5DMkII (and 7D etc) has a built-in buffer
so you don't need these expensive high speed cards like the Extreme Pro (90 MB/S) or arguably for the 5DMKII at only 3.9FPS, not even the Extreme (60 MB/S). You can take a 7D shooting RAW at 8 FPS -15 RAW max consecutively (2x the 5DMkII, albeit slightly smaller files (18MP vs 21MP)) and fire off almost the limit (b/t 10-12 RAW shots consecutively) with the slower 30 MB/S
cards due to the buffering. Now could you keep going and going without stopping? ...of course not...not with the 7D (let alone 1D) at least but probably can on the slower 5DII or close to it, haven't tested it that way. I have various SanDisk and Lexar mid-high cards including the Extreme Pro and now only buy the Extreme 60MB/S, not for the write speed as much as for the read speed when uploading images to LightRoom or other external disk. I also shoot outdoors a lot and in wet and sometimes very cold conditions and supposedly the Extreme is sealed a little better than the Ultra (or so they market it that way) The UDMA Cl.5/6 for read/upload is a value add, but in real-world scenarios with a 3.9 FPS camera (or a 10FPS camera for that matter), the write speed isn't required...save your money.