Use this one to change only font size not the name of the font
label1.Font = new System.Drawing.Font(label1.Font.Name, 24F);
you can change that using label property in property panel. This screen shot is example that
Use the Font Class to set the control's font and styles.
Try Font Constructor (String, Single)
Label lab = new Label();
lab.Text ="Font Bold at 24";
lab.Font = new Font("Arial", 20);
or
lab.Font = new Font(FontFamily.GenericSansSerif,
12.0F, FontStyle.Bold);
To get installed fonts refer this - .NET System.Drawing.Font - Get Available Sizes and Styles
You can also create a varible and then assign it for a text. It is cool because you can assign it two or more texts.
To assign a variable do that
public partial class Sayfa1 : Form
Font Normal = new Font("Segoe UI", 9, FontStyle.Bold);
public Sayfa1()
This varible is not assigned to any text yet.To do it write the name of the text(Look proporties -> (Name)) then write ".Font" then call the name of your font variable.
lupusToolStripMenuItem.Font = Normal;
Now you have a text assigned to a Normal font. I hope I could be helpful.
This should do it (bold as well);
label1.Font = new Font("Serif", 24,FontStyle.Bold);
Source: Stackoverflow.com