Input Field Widths

Question

I just created my account on ux.stackexchange.com. I'm a developer and don't have much experience in UX best practices. So I intend to start coming to this site regularly.

My question is really simple.

In a form with several input fields, is it better to define the same width for all fields (in order to maintain alignment), or to have different sizes for fields that are intended for less data?

For example:

Option 1:

 Last Name: [______________]
First Name: [______________]
       Age: [______________] <- this field is the same width as the rest even 
                                though it only supports 3 characters max. 

Option 2:

 Last Name: [______________]
First Name: [______________]
       Age: [__] <- this field is smaller and thus alignment with the rest is lost.

Which one is preferable? Keep in mind most of my forms will have at least 10 fields arranged in 2 columns, and maybe only 2 or 3 of them are going to be numeric or otherwise limited input fields.

Answer 1

Fields should generally be an appropriate size for their expected input, which may well mean that one field in a form has a different width to another field. The size of a field provides a useful clue about the information that should be entered into it. For example, if you're asking a user to input a postcode/zip, a field that is 20 characters long would be confusing because it creates the impression that you expect the user to enter something 20 characters long.

Answer 2

It's ok to have input fields of different length if the size of the input demands it. However you must avoid inputs of only slightly different lengths:

 Last Name: [________________]
First Name: [_______________]  <- not ok, even if that's the actual field length in your DB
       Age: [__]  <- ok: length contrast is sufficiently big