The dimensions of the assets should be correct. I compared a few GUI assets in the example project I have uploaded here and the one on my computer which looks fine. What's the issue you're seeing on your end? My guess is you might have changed something in the example project or in your own project files?
As mentioned in the product details, to achieve the look in the product photos you'd need to use the custom styles set up in "custom_gui.rpy" file in the example project you got with the purchase. It won't look like the photos without it. That file has all the styles applied to make everything look correct. The textbox.png file is correctly sized, but in the "custom_config.rpy" file the say window dialogue is set to a size of 1220 x 250 pixels. You can copy the file into your own project and alter the styles as needed. Read the comment at the top of it to see any other things you might need to change in your project files for the UI to look correct.