I'm trying to find a good default width for money input in my forms. If I assume that the amount people will put in is under 1 billion Euro, the width of the value, including decimal places and punctuation, would be around 100 pixels.
But how wide should the field at least be with regard to Fitts's law? How much additional space should I add to maximise clickability and keep the design minimal?
Also the user's expectation of how much they can put into the field plays an important role.
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Fitts's, after Paul Fitts.