  It has been conjectured that the tachyonic lump solution of the open bosonic
string field theory describing a D-brane represents a D-brane of one lower
dimension. We place the lump on a circle of finite radius and develop a variant
of the level expansion scheme that allows systematic account of all higher
derivative terms in the string field theory action, and gives a calculational
scheme that can be carried to arbitrary accuracy. Using this approach we obtain
lump masses that agree with expected D-brane masses to an accuracy of about 1%.
We find convincing evidence that in string field theory the lump representing a
D-brane is an extended object with a definite profile. A gaussian fit to the
lump gives a 6-sigma size of 9.3 \sqrt{\alpha'}. The level truncation scheme
developed here naturally gives rise to an infrared and ultraviolet cut-off, and
may be useful in the study of quantum string field theory.
