A perfectly respectable romance archetypes: think Tauriel and Kili or Mizuki and Caius. In both cases, the archetypes work in part because the issue is less about size or height and more about culture.
But a romantic relationship between a tiny person and a human really, always, about the difference in height.
In in my fan fiction, I took one of the Littles, Tom. He meets a human--much like Arrietty kept encountering humans--and even becomes attached. He takes himself off to the same college, where he makes a home for himself in the area behind the brick facade above a dormitory window, and--
I realized that for the relationship to move forward, I would have to resort to "science" (a la Honey, I Shrunk the Kids) or magic or dreams (a la Alice in Wonderland).
I'm not referring to a platonic friendship or even a platonic chivalrous admiration. Rather, a growing romantic relationship with the attendant physical relationship--unless it descends into salaciousness (See Gulliver's Travels)--needs to be on something of an equal footing.