fcc part 15.247 dsss
Well, it is a spreading issue. The FCC wants to make sure that your code is efficient enough to spread the energy out far enough to make your transmission less likely to jam someone. For instance, if you had a lot of unmodulated carrier leakage, your 6 dB bandwidth would be very narrow, and you would fail the test. So you start your transmission, you look at the screen of the spectrum analyzer, you peak the marker on the highest (should be the center) go out to the 6 dB down points and make sure they are at least +/- 250 KHz from the channel center.
I do not know what to set the RBW and VBW to. Maybe you can look up the FCC test reports of some other similar systems on the FCC site. You need to know the FCC ID number on the bottom of the competitor's parts in order to look up the test reports. I bet you will find one of them that details exactly how to do that test.