How can I change the text size when using a System.Windows.Forms.WebBrowser?

I am using a System.Windows.Forms.WebBrowser in my application.
I want the user to be able to change the text size, just like it can be done when using IExplorer.
(If you click the ‘View’ on the menu, and then ‘Text Size’, you can choose between 5 different sizes of text).
I tried the following:

Code:

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webBrowser1.Document.ExecCommand("FontSize", false, "2");

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But, it worked only on selected text, and value of font size can be from 1-7, so it’s not really the same.
Is there some other way?