Most likely the size of it, now that it's drained, is due to inflammation
and induration of the surrounding tissue as a result of the infection. That
will decrease pretty rapidly now that the absess is drained and the
infection resolved. That inflamed tissue will seep serum, and it will seep
blood because that inflammed tissue is fragile, and it's painful. Likewise,
that will decrease over the next several days as the inflammation resolves.

The packing is placed for two reasons - 1) to minimize oozing of blood for
the first couple of days after it's drained and 2) to hold the absess cavity
open so that the skin doesn't heal before the cavity is closed. As the
induration and inflammation go down over the next several days there will be
less oozing of blood. The cavity will remain open as the cavity granulates
( http://en.wikipedia.org/wiki/Granulation_tissue ) . That granulation
tissue will be kind of fragile and will ooze serum in modest amounts. At the
point that that granulation tissue forms, there will be no need to pack the
wound. To get to that point will take about 10 days - 2 weeks from time it
was drained. Once that heals and the skin is closed, I would pretty much
ignore it for a long time, until the surrounding tissue is completely
normal.

Complete excision of the cyst can wait until it re-forms (if it does) and
it's margins are readily definable. That will make for a more accurate and
smaller excision. Excision of sebaceous cysts of the back, in the location
you describe, are more difficult because the skin of the upper back is so
thick compared to other parts of the body.

HMc