Data Validation in textbox on userform

Hi, I have a userform where a user can enter a postcode in the textbox on the userform, and so the post code is entered in the table once the user clicks OK on the userform. I am just wondering is there a VBA code which can limit the user to only entering 2 characters for the post code, and if he/she enters more than two characters and clicks on OK on the uerform, then a message box pop ups warning that only two characters are allowed.

I am guessing a If statement is needed, yet because I am new to VBA coding I am not quite sure how to construct it. Can anyone please help me?