Is there anything you need to know before buying a grey card?

[Cullen]

Hi,

I need to get a grey card for a photography course I'm doing. Is there anything you need to know before choosing a grey card?

I have also seen some compact ones but my course recommends getting one thats 20 x 25 cm's. I think the portability of a compact card would be better for me, but is there any reason that you can't use a smaller card?

Thanks in advance

Cullen

[ameerat42]

I would have thought (ages since I've used one) that one that gives a pretty large area (for the lens/lightmeter) to read from would be "better", just in case you miss it and get a stray reading. Then, if the course recommends a larger one I'd be asking them, too.
Am.

[maccaroneski]

As long as it is big enough to fill your frame from a reasonable distance (i.e. not macro) when you are shooting it, I would have thought would be sufficient.