Zephyr is highly configurable and only the features you use are included in the binary, so memory usage depends entirely on the project. To get a ball park number it makes sense to pick an example project that is roughly what you want and check that. For instance if you want a rough idea of a simple BLE peripheral you can build the Bluetooth: Peripheral LBS sample, which will give you this memory usage (default configuration for the nRF52 DK):
Memory region Used Size Region Size %age Used
FLASH: 154084 B 512 KB 29.39%
SRAM: 26657 B 64 KB 40.68%
