You can modify boxes using CSS. If I had a question called RankThis and wanted to make the second input box 200 pixels wide, I could put this into the HEAD tag (or really anywhere on the page):
It seems like a bit of an odd request, though, because typically the label of the thing you are ranking is much larger than the rank value, i.e. Brand A is longer than a 2.
Regarding Paul's comment, my best guess is that we don't have a box to change the width because it's probably never come up before. I would imagine the vast majority of ranking question to be fewer than 10 items, so the default size works well for a single numeric input from the respondent?