What is the optimal size? Are certain brands better? What speed rating is preferred? Any other things to keep in mind?
Specifically, I am using a Canon t2i.
A class 6 card in theory should be more than enough bandwidth for HD (1080) video for the 550D (48 MBit). Unfortunately some manufacturers do not not produce as high quality consistent memory as they should and thus most HDSLR folks prefer to use class 10 cards which should have enough headroom that you'll never see the buffer bars.
I recommend not skimping on brand too much and going with a SanDisk class 10 - I have had no issues with mine on my T2i/550D or my 60D when shooting video.
Class 6 is sufficient. That camera can almost write as fast as that, though. You might consider a Class 10, which could be more useful in the future.
The size is really up to you and your take on "putting all your eggs in one basket." I have a few 32GB CF cards because I don't like fumbling with cards all the time. Basically, the more I'm handling my baskets, the more opportunity there is to break my eggs. That's just me.
Stick with the reputable brands for memory. Kingston and SanDisk are good. I'm sure there are some others.
You generally get what you pay for, but to take some of the pain out of it, there are benchmarks for memory cards online (although they're yet to test using the 550D). Whilst I've only experience of Compact Flash, I've always found Sandisk to be quick and reliable, and the benchmarks appear to back that up.
According to dpreview SanDisk Extreme III will get you almost unlimited JPEG burst size, and almost 2 secs of full burst shooting with RAW. 8 GB version should be quite enough.
Some cards are too slow to accept an HD stream, so you need one that is fast enough.
Since that camera uses the H.264 codec, it produces video at 5.5 MB/s at its highest-resolution. Some cameras used different codecs, so some models will even faster cards, particularly if using M-JPEG.
Look for something that has a WRITE speed of 10 MB/s or more and you'll be safe. Sandisk specifies their write-speeds in MB/s as opposed to Xs (you would need at least 66X to get 10 MB/s) which makes it easier to choose the right one with them.
Lexar Professional (20 MB/s) or Sandisk Extreme (30 MB/s) SDHC cards are currently top-of-the-line models with high durability. I use both of these as well as Kingston cards. High-end cards such as these come with a life-time warranty (at least here in Canada). I would not trust my images to cards which their own makers show less trust in them.
I recently got this Transcend 16Gb Class 10. I haven't tested it thoroughly, but if it really is a class 10 (or close, as I know that not all class 10 are equal), it's quite cheap for it's size.
I just bought this 32GB Class 10 card for my Panasonic TM700. I don't generally take 1080p60 video (only Class 10 cards are fast enough for that) but I may want to someday.
I didn't see much of a difference on Amazon, in terms of price, between Class 6 cards and Class 10 cards. In fact some of the Class 6's are more expensive than the Class 10's from the same manufacturer.
I'd say get a Class 10 card - even if your camera doesn't come close to using all the available bandwidth, the card will probably last long enough that you could use it in your next one.