The dimensions would vary depending on the form factor needed for the particular application, but a spool with a 9 inch by 9 inch spool with a 2 inch hub (which works for 1310nm applications) would hold about 120km
The fiber we're talking about has an outer diameter of 250µm (standard single mode) so it packs pretty densely; we can actually do a bit better using some more exotic (expensive) fibers and some of our proprietary tricks to packing a bit more in there, but the size I gave is pretty typical. The largest ones we have made fit in a 5U rack enclosure, but I can't remember the length we had on those offhand and some of the internal space was used for amplifiers.
