fullmatch and
pairmatch have a maximum problem size,
based on the number of comparisons between treated and
control units. For a completely dense problem, in which
every treated units is compared to every control unit
there are length(treated) * length(control)
comparisons. A caliper restricts which comparisons are
valid, disallowing matches of treated and control pairs
that are too far apart. A caliper can significantly
decrease the size of a matching problem. The
caliperSize function reports exactly who many
valid treated-control comparisons remain after applying a
caliper of the given width.caliperSize(scores, z, width, structure = NULL)