Recently I bought a Canon EOS 550D (T2i), and I need to buy an SD card of speed class 10.
What size should I get? Is there some limit in terms of camera support? Should I go for 8, 16, 32, 64GB, or in smaller capacity cards to be able to download one while shooting with the other?
Your camera supports SDXC cards which have a theoretical size limit of 2 TB, so it supports any SD, SDHC or SDXC memory cards on the market.
The difference between SD, SDHC and SDXC is maximum capacity. SD cards are limited to 2 GB. If you shoot video, you will notice that limit quickly. For images, it depends on how much you shoot and how often you offload. SDHC cards go up to 32 GB which gives space for thousands of photos. Your camera manual has a table near the end to let you know how much photos and video fits in different sizes.
Pick a size you are comfortable with. Some people decide to have one huge card and never swap and others like smaller chunks so that if one fails, you still have other cards and you lose less. Even top-notch cards can fail, so I strongly recommend more than one.
The card rating is prominent on most packages except for the slowest cards :) If you need a class 10 then you need one which says 'Class 10' or at least 10 MB/s or at least 66X. Some manufacturers use different measuring systems. Today you can get cards up to 1000X or 95 MB/s which is faster than your camera can take advantage of.
I use a 32GB card, but I feel that was a mistake. If I could do it again I would've purchased multiple smaller cards. If I had more than one card I wouldn't suffer from a single point of failure in the SD card. If one failed I'd lose those photos (if any) and then I could still use the backup.
Also, if you do not frequently empty the card then it takes longer and longer to grab the new photos. I had thought that with 32GB I could just keep taking pictures forever (I don't shoot more than 1000-1500 per year and the card holds well over 1000 on my 60d in the largest RAW format), but it got to the point that it would download all the old pictures to get "previews" and so I started erasing all images after every transfer so I wouldn't spend more time downloading than actually taking pictures.
And also, if you go super big for your needs you'll over pay. Find what your needs are for the near future (est. 1 year) and then buy more in the future because it'll only get cheaper. And having more memory cards isn't a bad thing if you can keep them empty (no good having a bunch of extra full cards).
The camera has no capacity and there is no maximum size you have to worry about. The camera just writes to the SD card until it fills up. If you can afford (and want) a 64GB card, go for it. There is no speed difference between cards of different sizes.
SD cards come in SD, SDHC and SDXC versions, with progressively faster write speeds. Go for at least an SDHC as you may find that the standard SD card will be a little slow, especially if you're shooting raw.
If you envisage yourself being in a situation where you will need to keep shooting while transferring photos, then by all means get two cards, but otherwise it's not really an issue as transferring only takes a few minutes.
As with any purchase of this kind, get the biggest, fastest version you can afford and you won't go far wrong!
