For WinForms, you need to set the 'Font'property of the Label with new size parameters instead of setting the Size property directly. The Size property is read-only in Winforms.
Heres the code for it
float size = 100;
label1.Font = new Font(FontFamily.GenericSerif, size);
Use the Font(....) overloads as per your requirement
Try to be more clear on your question next time. Cheers !!!