That function tells you how much space there is left between the top of the stack and the bottom of the heap. If that's really -114, as I surmised earlier, your heap and stack are trying to use the same memory. Very bad things will result.
Look at progmem - see what constant strings you can keep in flash until they're needed.If you need buffers, make sure they're no bigger than needed - not 1024 just in case. This is rather old school compared to anything you might do on a PC these days
avr-size myproject.cpp.elf text data bss dec hex filename 7240 598 380 8218 201a myproject.cpp.elf