Expand properties in solution explorer of the project. Open Settings and set default font size and other defaults for application.
In the constructor of each form add following line:
//set fonts
this.Font = Settings.Default.appFont;
You can change/set the value of Settings.Default.appFont.
If you want to access object of all screens/forms then you must create CollectionBase for each form. Collection can be used to assign fonts to all screens from one class. check this link http://support.microsoft.com/kb/815707
Also Application.OpenForms collection could also be helpful.