A Field's Validators properties allow you place restrictions on its values, including the length of the data.
When an end-user provides information for the Field, the value will be checked to see it is within these set limits.
Fields also have a Display Length property that lets you set how many characters wide the Fields corresponding input field box will be. This doesn't restrict the number of characters that can be entered, just hides any characters longer than the this.
Fields with the Text Area Display Type also have the 'Number of rows' property that allows you to set the height of the input box.