How to change form width and height programmatically in C#...

After the Size is defined, change either the form height or width by using the Width or Height properties.
The following code example shows the width of the form set to 300 pixels from the left edge of the form, whereas the height stays constant.

Form1.Width = 300;

-or-
Change Width or Height by setting the Size property.
However, as the following code example shows, this approach is more cumbersome than just setting Width or Height properties.

Form1.Size = new Size(300, Form1.Size.Height);