Generally speaking, we recommend uploading images that are the size of the container they're filling, or at most 2x that physical size. For example, if you've determined that your Project Cover Images are displaying at 350px wide, you'll only need to upload an image that's 700px wide. Similarly, if you've determined that your Project Container will max out at 1600px, you'll want to upload an image that's between 1600px and 3200px. Keep in mind that the size of the images that you're uploading will also affect the speed at which your site displays.
To determine how large the viewable space on your Portfolio is, please read this FAQ.
Currently, individual project images (including Project Covers) can be up to 50MB. Audio and video uploads are limited to 1GB per file.