Go into yourForm.Designer.cs
Scroll down to your textbox. Example below is for textBox2 object.
Add this
this.textBox2.AutoSize = false;
and set its size to whatever you want
this.textBox2.Size = new System.Drawing.Size(142, 27);
Will work like a charm – without setting multiline to true, but only until you change any option in designer itself (you will have to set these 2 lines again).
I think, this method is still better than multilining. I had a textbox for nickname in my app and with multiline, people sometimes accidentially wrote their names twice, like Thomas\nThomas (you saw only one in actual textbox line). With this solution, text is simply hiding to the left after each char too long for width, so its much safer for users, to put inputs.