Graphic style guidelines

Hello,

I'm new to creating my own graphics and would like some advice.

I'm editing together a video that is primarily text graphics and graphics will be created in Photoshop using a PowerPoint presentation as the starting point.

I converted all the PPT to PSD files but am finding that there's too much information on the resulting files I will use in the video.

Is there some sort of rule of thumb or principle guiding how much text a single full screen graphic should have when the end result is either a DVD or video delivered via the web?

Thanks in advance for your advice.