I have had a few different gray cards. I've not had much success with a grey card. It usually results in too yellow images for me. It's definitely better than nothing and worth a shot. I really never use anything to set custom white balance any more...I just do it manually in post. I can still batch process from my first corrected image. Hmmm....maybe I should try setting custom balance again, it's been a while and I've gotten lazy.
ANYWOO......In general, if you're using the gray card for custom camera white balance bigger is better. If you're using to correct white balance in post processing, then the smaller ones are fine. Just make sure it's for digital as digital and film cameras have use different neutral grays. I don't know too much about it, but I remember reading about it a few years ago. I like the cards that have gray in the middle, black on one side and white on the other. It's nice to be able to click on the white point and the black point and the gray point in post processing for even more accurate color, exposure and contrast.
Just to add...this is what I actually do use for my headshot sessions. I find it too cumbersome for portraiture sessions, so I just eyeball it in post processing then, but for headshots when dead accurate color is more important I use this. MUCH more expensive than a grey card, but I LOVE it! Amazon.com: X-Rite MSCCPP ColorChecker Passport: Camera & Photo
I would opt for the lastolite for convenience.
Amber, the product you linked to looks awesome. We can't really afford that, but I'd sure like it!
I figure for the price, a gray card is worth a try, because I think it'd give me a closer starting point than I sometimes have. I always check my WB setting before shooting anything, but sometimes the light is challenging and nothing seems quite right. But I like the suggestion about the whites and blacks and am thinking of going for something like this instead: Amazon.com: Fotodiox Hand Hold Collapsible Disc, Tri-Fold Reflector + Gray Card/White Balance Digital Target Combo: Camera & Photo
The convenience and accuracy of a larger gray source, like El pointed out, would be great; and this one has white/black too. I figure it'd be easy enough to get a portrait subject to just hold it up for a reference shot, and go from there.
What I'd really love is something other than GIMP so that I can batch-correct color once a shoot is done. But, alas, one thing at a time.