Where is the zoom?
You will find the zoom slider on the bottom right corner of the Excel window. You can drag it to the right to increase it gradually (thus making the text bigger), or to the left to decrease it.
You can also use the small "plus" and "minus" buttons to change it in predefined steps.
When should you use the zoom?
Sometimes when working with big spreadsheets, it is more convenient to look at your data "from a distance".
It is mostly useful when your data has only one or two columns (or rows) slipping outside the screen. Setting the zoom to 75% can enable you to see all your data at once, yet the text doesn't become too small to read.
The opposite case - when you should "zoom in" will occur when you wish to focus on a small piece of your spreadsheet, or the text's font is just too small and you don't wish to mess with changing font sizes and all the adjustments involved with it.