Save Good vs Export BadYou've spent your time creating the perfect peice of artwork - its the right size and its 300dpi for good measure, this design is going to look sick and your customers are going to love it.
But wait you've done a test order and the print has come out larger than you anticipated, you've checked your original PSD and yet its the correct size and 300dpi.
The problem? When you went to save in Photoshop to create your 300dpi .png - you used the File > Export > Quick Export to PNG.
It might seem quicker but this method creates your PNG at the same size (pixels) but at 72dpi. That as a result actually makes the print file dimensions so much bigger, a 14cm print actually becomes a 58cm (ish) print file.
It might have a few more steps in it but File > Save As is the better way to go when it comes to getting the correct size artwork which is ready to print.