I have a heap_size problem compilating freertos for the pic18f8720.  The C compiler shows the warning message ‘cannot fit the section’.  The only solution that I found was to fit the heap_size at 251 bytes! The pic18f8720 has 4kbytes of RAM.

You need to modify the linker script to create a block large enough to hold the heap. Take a look at the linker script 18f452.lkr found in the demo/PIC directory of the download.  The block named BIG_BLOCK is where the heap goes. Also take a look at the thread: https://sourceforge.net/forum/forum.php?thread_id=1255600&forum_id=382005 which notes a get around for variables that straddle the block boundaries. Regards.