You are using length, which just gives the number of elements that is used to represent a graph. Every igraph has “length” 10. Instead, use vcount, ecount, or summary. For example:
> g <- make_full_graph(50)
> summary(g)
IGRAPH a3a4dde U--- 50 1225 -- Full graph
+ attr: name (g/c), loops (g/l)
> length(g)
[1] 10