RF24Mesh | OSI Layer 7 Mesh Networking for RF24Network & nrf24L01+

If you add your library in the arduino IDE as it is described here it just consists in linking another project to your library functions. It's not a static library (see static and dynamic libraries). Then I think it's not necessary to worry about its size since the compiler will embed your library functions only if you use them.

Try opening any example (AnalogReadSerial), compile it. Then Sketch->Add a library->SPI. Compile it again, the size does not change. Try to call SPI.begin() in the setup function, the size increases. Add a call to SPI.setBitOrder(MSBFIRST);, the size increases again. Add another call to SPI.setBitOrder(MSBFIRST);, the size increases again, but not by the same amount since it contains only one setBitOrder definition and two calls to the setBitOrder function.

That's not exactly true for all libraries since some constructs could force the compiler to embed some code or allocate memory even if the variable is not used (see for instance volatile variables).

So regarding your size issue, you'd probably only need to use one #define MASTER, write the master code in setup and loop functions surrounded by #ifdef MASTER and the slave code surrounded by #else...#endif. The compiler will include the function definitions that both master or slave use.