The size (length) of a field is a matter of usability in that it can provide a valuable affordance to the user.
Take the following example:
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Even without labels you can probably guess what the fields are:
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Obligatory Wroblewsky quote (The above example is from Ch. 5 of his book Web Form Design (p. 73-74)):
Because affordances provide valuable clues about how to structure answers, people will naturally consider those clues when they are thinking about how to fill in an input field. If that input field is arbitrarily sized, you may leave people thinking about what that means for no reason. [...] Said another way, if this input field is long but my answer is short, have I misunderstood the question? Best not to make people think too much and instead use a consistent length for all fields that don't benefit from clear affordances.
With regard to your question, no, it is not useful to have all the input fields the same width.
Depending on the context of the data the user will be entering, it may make sense to have variable widths, though those widths should not be arbitrary. Rather, the field widths should instead give the user a clue (affordance) as to what should be entered in the field if appropriate.