Decrease:
Be aware that there is a minimum row height (that depends on several factors including font size, bar styles, bar text you have chosen).
1 row: place your mouse cursor on the row divider line beneath the ID number. The cursor will change to a bar with an up and down arrow. Click and drag up. 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.
n rows: select the rows you want to resize (they do not have to be contiguous) and then follow the instructions for 1 row.
Text on Gantt chart: again, Project will impose a minimum height for the row depending on the factors I mentioned above.
Smaller paper: File | Page Setup | Page-tab and you can adjust the %Normal Size. This will, of course, impact the apparent size of the text. You'll have to experiment to see if you can get items on to 1 page in a large enough display to satisfy your needs.