What are the distinctive structural features of single- or double-stranded xRNA, or where can I find them?
The only paper I found that examined the question is "Structural, Dynamical, and Electronic Transport Properties of Modified DNA Duplexes Containing Size-Expanded Nucleobases". They state


  The results confirm that the structural and flexibility properties of
  the canonical DNA are globally little affected by the presence of
  benzo-fused bases. The most relevant differences are found in the
  enhanced size of the grooves, and the reduction in the twist.


They also have a picture comparing the structures of conventional DNA and xDNA:



So while the basic structure seems similar, the difference in diameter and twist look rather large to me.

This is for xDNA, I would assume that xRNA behaves comparably if double-stranded. It would have to somehow fit the larger nucleobases into the helix like xDNA. And the structure of single-stranded RNA is variable, helical parts would probably look similar to xDNA, hard to say how the modified nucleobases would affect tertiary structure.