get connected components using igraph in R

You can use the results from components to subset your nodes according to the component size.

library(igraph)

# example graph
set.seed(1)
g <- erdos.renyi.game(20, 1/20)
V(g)$name <- letters[1:20]
par(mar=rep(0,4))
plot(g)

# get components
cl <- components(g)
cl
# $membership
# [1]  1  2  3  4  5  4  5  5  6  7  8  9 10  3  5 11  5  3 12  5
# 
# $csize
# [1] 1 1 3 2 6 1 1 1 1 1 1 1
# 
# $no
# [1] 12


# loop through to extract common vertices
lapply(seq_along(cl$csize)[cl$csize > 1], function(x) 
                                         V(g)$name[cl$membership %in% x])
# [[1]]
# [1] "c" "n" "r"
# 
# [[2]]
# [1] "d" "f"
# 
# [[3]]
# [1] "e" "g" "h" "o" "q" "t"