Gennaro,
There are a few approaches. The easiest approach is to change the magnification in the lower right corner (default 100% -- I find that 150% is usually good depending on the monitor size). This gets saved with the notebook but it must be done manually one time.
Another approach is to create a stylesheet (or edit/replace the default stylesheet). The reason this is a bit more involved is because there are many styles that are pre defined with fonts, coloring, background color, etc. You can change any or all but in a notebook you may use several (For example, I tend to use "Input", "Text", and "SubSection" often but you may use others). You can also look at some of the built in stylesheets to see if they better suit your needs. Go to Format->Stylesheet to get started. I hope this helps.
Regards