•.Open Calibre
•.Import the book you wish to convert into Calibre if it's not there already.
•.Select the book you wish to convert
•.Click the Convert button on the toolbar
•.Select your desired output format using the dropdown in the top right
•.In the left hand pane, select “Look & Feel”
•.At the end of the “Font size key:” line, click on the wizard button ()
•.If you don't see the “wizard button”, click in the Font size key text field, press TAB and then press SPACE.
•.At the bottom of the font rescaling wizard, click “Use default values”
•.You will see a base font size and a font size key. The font sizes in the key can be edited to suit your tastes, but I do recommend you keep it in the small to large order.
•.Click “Ok”
•.In the Convert dialog click “Ok” to perform the conversion.
•.Test it out.