This is confusing; exactly what window are you referring to? Do you mean the reading pane when you have an email open or something else? How are you accessing your email---------on a computer using a browser and the webmail UI, or on a tablet or what?
On a Windows machine, hold down the Cntrl key, then tap the - key to reduce it, the + key to increase it, or the 0 (zero) key to return it to the default. See if that's what's going on.