Seasoned print designers know all about DPI, but it is difficult to convince design students unless they are shown an example as to what can go wrong. I made this sheet where the same image is shown at the same size except in 300dpi, 70dpi and 30dpi. I also included a badly compressed JPEG so that students will know what to look for.
The correct DPI to use will always depend on the printer that is being used so ask! If the DPI is too small then some blurring and pixellation will occur. This can be seen where the text is not clear and where there are square step blocks.
The poorly compressed JPEG shows how the textural detail in the wall and sky are flattened out and bruising artefacts appear where there is a change in contrast (e.g. around the text).
Having a poorly compressed JPEG and DPI that is too low is a double blow to the quality of the output image.
Feel free to share the document. Link: The DPI Problems PDF
maximum_display_size_in_inches = pixel_size divided by the DPI
(there are 2.5cm in an inch. So, approximately 4 inches in 10cm)