Before you create a large puzzle, you should make sure that the image in your source contains enough detail (i.e. enough megapixels). You can use a file utility or photo viewer/editor to find out how many pixels wide and high your picture is when not compressed by JPEG or other image format. As a rough rule of thumb, allow one megapixel per 1,000 pieces. That will give each piece about 1,000 pixels or more than 30 by 30 pixels, making the pieces reasonably sharp at the edges and clear to see when you zoom in. You can work with fewer megapixels, but it may be hard to distinguish and recognize pieces during play.
When you create or restart a large puzzle, it may be best to avoid using the bevel and shadowing features of Palapeli (see the General settings section), because they make loading slower and highlighting harder to see when the pieces in the view are very small. More prominent highlighting appears when shadowing is not used.