How to change the picture to 16* 16 in photoshop?

Change the parameters in Image-Image Size on the menu bar, as shown below. Your 16* 16 should mean pixels (I guess it's a small icon). Please ask the landlord clearly. Then, when you change it, you need to change it to other units, such as centimeters and millimeters ... just left-click the unit option (as shown in the figure, pixels) to change it. The landlord's request is within 2M. At this point, you should modify the value in the "Document Size" column. After modification, select File-Save As PNG and JPG. I wish the landlord a successful refit.