For digital sales (stock or downloads), when defining a price value, you can indicate that the purchased image or video must be delivered in a certain format (the Description does not matter).
You can define a size in KB/MB, in which case the image will be resized to exactly your specifications (1 Mpixel = 3 MB).
Alternatively, you can define a pixel size, as well as whether the delivered image should fit within that size, or should be just big enough so that the defined size can be cropped without resizing.
For generic web/email use, considering that a standard computer display is around 1500 pixel wide, 800 pixel-wide images are usually a good compromise.
For example, if your original image width/height ratio is 3:2:
Note that PhotoDeck does not display prices for sizes that are bigger than the uploaded image file.
PPI (pixel per inch) is a figure that indicates to a printer what physical size the image should be printed to based on its pixel size. It is just a metadata figure, that has no one-to-one relation with the size or quality of an image. For example, a "300 ppi file" (commonly called "300 dpi file"), says nothing about the quality unless a size is associated with it -- a tiny 30x30 pixel image can be printed at 300 ppi, it will just come out as a 0.1 x 0.1" print.
What really matters is the pixel size!
Note that we indicate, for your clients convenience, how big a specific image size would print if it where to be printed at a "standard" 300 ppi resolution, but it doesn't mean anything more than that.
Similarly, video pricing profiles allow to define options for delivered sizes, as well as encoding format.