Constant ROM_SIZE 

pub const ROM_SIZE: usize = _; // 12_288usize

ROM_SIZE is not specified in tk1 memory model. Instead, this is the maximum allowed size that is made available in EBR (Embedded Block RAM) in the verilog model. See hw/application_fpg/core/rom/rtl/rom.v in the tillitis-key1 repository containing the hardware, fpga and firmware content.

rom.v (verilog):

 "Max size for the ROM is 3072 [32-bit] words, and the address is 12 bits to support ROM
 with this number of words."