Qfyilyi

I created a .NET Framework 4.0 winforms project and added a "tab" as you can see in the following picture:

From the View Designer both tabs (Profile and Features) looks like they fit the rectangle window, but when I run the program, the "Profile" tab looks the same but the "Features" tab suddenly does not fit the rectangle anymore and looks like that:

I don't want to give the user the option to enlarge the window.

I am wondering why it looks like it fit the rectangle window but when I run the application, it does not.

This is the "Features" tab from the MainForm:

        this.TabPage2.BackgroundImageLayout = System.Windows.Forms.ImageLayout.Stretch;

        this.TabPage2.Controls.Add(this.listBoxShowFriendsBirthday);

        this.TabPage2.Controls.Add(this.buttonShowFriendsBirthday);

        this.TabPage2.Controls.Add(this.label1);

        this.TabPage2.Controls.Add(this.monthCalendar1);

        this.TabPage2.Controls.Add(this.postTextBox);

        this.TabPage2.Controls.Add(this.pictureBoxEvent);

        this.TabPage2.Controls.Add(this.listBoxFetchEvents);

        this.TabPage2.Controls.Add(this.buttonFetchEvents);

        this.TabPage2.Controls.Add(this.postButton);

        this.TabPage2.Location = new System.Drawing.Point(8, 39);

        this.TabPage2.Name = "tabPage2";

        this.TabPage2.Padding = new System.Windows.Forms.Padding(3);

        this.TabPage2.Size = new System.Drawing.Size(1976, 806);

        this.TabPage2.TabIndex = 1;

        this.TabPage2.Text = "Features";

        this.TabPage2.UseVisualStyleBackColor = true; 

I suggest you to use Dock and Anchor properties of controls. This way you can also allow to user resizing the window.

Dock:

Anchor:

See - How to: Anchor and Dock Child Controls in a FlowLayoutPanel Control

To prevent the user from rescaling their window:
c# how to prevent user from resizing my application window?

but aside from that, I'd suggest, (just like ikram said) to use docking, that way you're certain your lay-out will remain, no matter the resolution/screensize the user has.