You can't set a text box to a specific pixel height without selecting MultiLine - in which case it expects multiple lines as input, so it uses the ENTER key. Single line TextBoxes always resize to fit the Font size specified (Which makes sense, or the text in it would not be readable). So, You can chnage the Font size:
Set the TextBox Font property:
myTextBox.Font = new System.Drawing.Font("Microsoft Sans Serif", 4F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
Will use a smaller textbox - I think you are making a mistake though. Most users adjust the monitor size to fit their eyesight, and a much smaller font may not be readable to them (or anyone else).