In the page layout tab, check the page scale in the scale to fit section. It should be around 70% to 100% for readability (depending on your contents and formatting).
Another cause: you probably are printing all the cells in 1 page. If so, split them to multiple pages. In the page layout tab and under 'scale to fit' section, set the width and height to 'Automatic' and adjust the scale to around 80% or more (varies from person to person. I can handle up to 65%).
Check the print preview before printing to see if you can read them.