C# ,The text in the Text Box or texts are shrink in the text box

I assume what you describe is what you expect. This is not how TextBox works. There is not a simple way to achieve this behavior. Manipulation of the font as contents of the TextBox is quite possible, but it's nearly impossible to adjust required font size, as exact string size measure is pretty inaccurate, due to the complex nature of text rendering.

More importantly, such behavior would make application nearly unusable. Letters should not shrink; it would make them nearly invisible. The font should change only by explicit command from the user. For a good example, look how modern versions of major Web browser handle this.

—SA