It is possible to zoom in/out the puzzles you are working on. This option is available for puzzles starting with 140 pieces.
When you start a puzzle, it automatically opens enlarged at one point. However, you can change it and make the puzzle smaller or enlarge it again by pressing the Scale button.
Please note that there is no option to change the enlarged area of the puzzle, it is done automatically. You will be moved to the next part only when you fully complete the first part.