How much can we downsample by? and how much can we upsample by?
Nyquist. If your signal is real and has a BW of B = 100 Hz (meaning DC to 100 Hz), then the most you can downsample by is 2, in order to obey Nyquist and have a sampling rate that is at least 2B = 200 Hz.  Note you will typically want to sample more beyond this limit to allow room for reasonable filtering.