Most class files in LaTeX will come with different options, such as 10pt font size, or a5paper. If you specify these options global option, you will generate an error message which looks like main.
Some classes, such as scrbook are capable of accepting any font size as an option. Whenever using a new class, you should always check what font options are available in the class documentation. For a wider variety of global option size options, one should consider the classes in the extsizes package.
This package provides you with classes such as extarticle, extreport, extbook and extletter. These classes allow font options of 8pt, 9pt, 10pt, 11pt, 12pt, 14pt, 17pt or 20pt.
If you would like to learn more about different font sizes in LaTeX, check out our documentation.
Using an option not specified by the class: Another common cause of this error message is when an option is used which is not specified by the class. This is a mistake, because the Thesis class has named this option as twosided. Options such as this can change from class to class.
For this reason it is important to check the class file as well as any documentation which comes with it. This way you will know what options are available in that class.