No, you can not change the font size within a MessageBox. The MessageBox font size is defined by the user's operating system.
The only way you can do this, is by creating your own custom Message Box:
How To Make a Custom Message Box
To change the Windows Forms form font, just use the Form's Font Property:
public partial class MainForm : Form
{
public MainForm()
{
InitializeComponent();
this.Font = new System.Drawing.Font(
"Microsoft Sans Serif",
24F,
System.Drawing.FontStyle.Regular,
System.Drawing.GraphicsUnit.Point,
((byte)(0)));
}
}
Replace "24F" with the desired font size.