libzxid contains thousands of functions and any given application is
unlikely to use them all. Thus the easiest, safest, no loss of
functionality, way to reduce the footprint is to simply enable
compiler and linker flags that support dead function
elimination.
On gcc you should investigate compilation with "-ffunction-sections -fdata-sections" and linking with "-Wl,--gc-sections".
If you need to squeeze zxid into as minimal space as possible, some functionality trade-offs are supported. I stress that you should only attempt these trade-offs once you are familiar with zxid and know what you are doing. The canned install instructions and tutorial walk thrus stop working if you omit significant functionality.