How can I fix the form size in a C# Windows Forms application and not to let user change its size?

Check this:

// Define the border style of the form to a dialog box.
form1.FormBorderStyle = FormBorderStyle.FixedDialog;

// Set the MaximizeBox to false to remove the maximize box.
form1.MaximizeBox = false;

// Set the MinimizeBox to false to remove the minimize box.
form1.MinimizeBox = false;

// Set the start position of the form to the center of the screen.
form1.StartPosition = FormStartPosition.CenterScreen;

// Display the form as a modal dialog box.
form1.ShowDialog();

Try to set

this.MinimumSize = new Size(140, 480);
this.MaximumSize = new Size(140, 480);

Minimal settings to prevent resize events

form1.FormBorderStyle = FormBorderStyle.FixedDialog;
form1.MaximizeBox = false;

Properties -> FormBorderStyle -> FixedSingle

if you can not find your Properties tool. Go to View -> Properties Window