The full range of the bands are 2 to 4 GHz (S), 8 to 12 GHz (X), and 27 to 40 GHz (Ka); see Wikipedia IEEE radar bands for the whole list.
DSN uses only a small fraction of those ranges. For example, in S band, DSN stays inside 2025 to 2120 MHz and 2200 to 2300 MHz, but doesn't fill it everywhere (part of that is unused by the Madrid station due to interference). See JPL 810-005
201, Rev. D for the rest.
Even that is a gross oversimplification, because all of these bands are licensed for multiple uses. What the S band portion really looks like, at least in the U.S., is this:
, taken from this chart.
The individual channels into which each DSN band is divided are expressed by a page of formulas which may be found in the same JPL document I linked above. It begins with:
The S-band downlink center frequency (Fch(14) = 2295 MHz) is used to
derive all entries in the tables using the expressions Fch(n) =
(n-14)*(10/27) + 2295 MHz, rounded to the nearest Hertz where Fch(n)
is the center frequency (in MHz) of channel n rounded to the nearest
Hz, and the ratio 10/27 is the spacing (in MHz) between the centers of
two adjacent channels.
10/27 MHz means each DSN channel is centered 370 kHz apart, but that still doesn't tell us how much of that width is actually occupied by the modulation in use. A different JPL document in the same series, 810-005 206, Rev. C, says that is user-selectable, using BPSK rates of 4 kbps to 256 kbps, and a wide variety of error correction encodings.