This is more of a general graphics question moreso than Photoshop specific. I am constantly dealing with clients who supply me with their logo. It's for use on broadcast television. Usually I'm stuck working with a jpeg image. The question I am always asked by the client is "How big does it have to be?"
I know it depends on how much screen space it's going to take up and how it was compressed, but is there at least a ballpark answer I can be giving? Usually I just say "the bigger the better" and they send me a little 10k jpeg.
Assuming the logo will be fullscreen, what guide of reference can I give that will make some sort of sense? I'll often say "over 50k would be good" but even that's a crapshoot.
Standard for Broadcast in the US is NTSC which is 720 pixels x 480 pixels so that is generally the way to go about describing the size.
A lot of people shoot out they want a certain DPI but that doesn't make sense because you can have a tiny image at a high DPI or a super large image at a small DPI. So if you have the minimum pixel requirements you should be fine. If you want to throw in that you want a high quality image is good as well in case you're dealing with someone who might know about exporting graphics.