In Le Guin’s The Dispossessed, how would you analyze the wall on Anarres in relation to “Saussure’s Redefinition of a Word"? What is the “Sign,” “Signifier,” and “Signified” in relation to this symbol? Secondly, how do Le Guin’s various interpretations of this wall illustrate Derrida’s idea concerning the arbitrary nature of “signifieds”?

According to Saussure, a "sign" is made up of a "signifier" and a "signified," and the relationship between the sign and real thing signifying it is capricious; in other words, these relationships are not organic. Our modern interpretation of this concept refers to the signifier as the physical or material form of the sign (something that is tangible and accessible through the five senses). Saussure, however, interpreted both the signifier and signified as matters of form and not substance. Words have no inherent value, which is exactly why they are useful. 
So, in the case of the wall on Anarres in Ursula Le Guin's The Dispossessed:
the wall itself is the referent
the word/sound "wall" is the signifier

the concept or meaning of "wall" is the signified

The above signifier and signified comprise the "sign."
Derida emphasized the arbitrary nature between these components, with "the organizing principle for the whole of linguistics, considered as a science of language structure" and "no fixed universal concepts or signifiers." There are so many different descriptions of the wall here to, thus, satisfy this concept; it is an "idea of a boundary" but "the idea was real." The wall can operate in the realm of the "signified" in a variety of ways which are wholly up to the individual who beholds it or the social conventions of the area; this is relative. It can be an obstacle to freedom or a safe enclosure; it can keep something in or out; it can be both literal and mental; etc. Thus, the wall cannot operate as one universal thing since there are (again) no universal concepts or signifiers and because it is a component of an "arbitrary way of organizing and conceptualizing the world." 
https://newderrida.wordpress.com/category/derrida-and-saussure/