RE: MRF24J40MA zigbee stack question
Large software stacks are pretty much the reason why a few of the PIC families have 64/96/128kB of flash. The short answer about what to do when your PIC of choice doesn't have the necessary flash (or SRAM) is "Don't do that".
Also, if you're not going to use a PIC for your MCU (and use an FPGA in your case, as you'd mentioned), you won't be using the Microchip stack anyways. However, the size of the stack (subset) that you need will give you a ballpark guesstimate as to how much codespace you'll need for your FPGA implementation.