Please move the question to Products. "Marketing Central" should be (ideally) for general questions across the discipline, as opposed to Marketo product support.
As for an answer, there doesn't seem to be anything special here. Add a custom field, add it to the form. The most efficient way to store a size range is as an integer: either representing the upper bound of the range (i.e. 1-5 is value 5, 6-25 is value 25) or representing a "size tier" (1-5 is value 1, 6-25 is value 2, etc.). But you could also just use a string field if you want. On the form, make it a Select.