We cannot directly use a micometer on the sponge since it will squeeze it.
However, what can be done is using a travelling microscope to focus it on the edges of the sponge and then to take readings.
It is very impratical to use the micrometer here.
An alternative which still seems veey unsuitable would be to fix (harden) the sponge using a resin and then measuring the dimensions with the micrometer but here the sponge will be lost foreever.