This will, of course, impact the apparent size of the text.
Smaller paper: File | Page Setup | Page-tab and you can adjust the %Normal Size. Text on Gantt chart: again, Project will impose a minimum height for the row depending on the factors I mentioned above. N rows: select the rows you want to resize (they do not have to be contiguous) and then follow the instructions for 1 row.
To make it the smallest possible (subject to the constraints I mentioned above), simply drag the cursor up as high as you can (until the row seems to have 0 height) and release the mouse button. The cursor will change to a bar with an up and down arrow. Be aware that there is a minimum row height (that depends on several factors including font size, bar styles, bar text you have chosen).ġ row: place your mouse cursor on the row divider line beneath the ID number.