how can i know whether the CFM is big enough for my nios project in max10 ?

The hardware configuration size is constant for any given FPGA (unless compression is used). It is 0x3f68f in your case. You can use all available RAM for software and data. Eclipse doesn't know or care how large your object code is. If you have the memory and section sizes set correctly in the linker script, any built that doesn't fail will fit in memory. At runtime, check return codes for malloc etc to monitor the heap. See the sofware guide for info on detecting stack overflow.