It seems this is initialized to numeric by network.initialize, e.g.
require(network)
nw <- network.initialize(100000L, bip = 60000L, directed = FALSE)
class(network.size(nw))
# [1] "numeric"
network.dyadcount(nw)
# [1] 2.4e+09
However, add.vertices and delete.vertices appear to change it to integer, which can e.g. lead to network.dyadcount overflowing:
require(network)
nw <- network.initialize(100000L, bip = 60000L, directed = FALSE)
delete.vertices(nw, 1)
class(network.size(nw))
# [1] "integer"
network.dyadcount(nw)
# [1] NA
# Warning message:
# In nactor * nevent : NAs produced by integer overflow
require(network)
nw <- network.initialize(100000L, bip = 60000L, directed = FALSE)
add.vertices(nw, 1)
class(network.size(nw))
# [1] "integer"
network.dyadcount(nw)
# [1] NA
# Warning message:
# In nactor * nevent : NAs produced by integer overflow