Galaxy filaments can be comparable in size to what you're talking about. Some of the largest ones exceed 1 billion light-years in length, including
Filaments are expected to form from density fluctuations in the early universe, and subsequent interactions with dark matter.
Large quasar groups are claimed to be even larger, although there are some disputes as to whether some of these are true structures:
Observations suggest that large quasar groups are the progenitors of galaxy superclusters (as they occur at redshifts of $z\simeq1$). They do seem to cluster in similar densities as supercluster members, making that evolutionary path all the more appealing.
It's often argued that structures on this scale - say, 1 billion light-years - violate, or come close to violating, the cosmological principle, which expects effective homogeneity at these scales. This has partially contributed to arguments that structures like large quasar groups should not exist - and, indeed, that observations of them may simply reflect random clustering or so-called pseudostructures.