Import prep Photoshop files into AE
So when working in Motion or Video the DPI value has no importance ( i.e: 72 or 300 dpi is the same), what will really affect your artwork is the PIXEL DIMENSION of your source file.
The pixels needs to be be enough or event bigger then the AE screen dimension your working on.
i.e: Your AE comp is 1280 x 720, then make sure your photoshop file is the same.
If you plan to have a zoom for instance, then make sure the files in bigger in photoshop. i.e: 1480x 920 for example.
However do not make your photoshop files TOO BIG as it will suckup all your AE memory.
Source info: http://archive.divergentshadows.com/dv/ae/notes/photoshop.shtml