How to know the code size ?!

You can run nios2-stackreport on your *elf file. It should give you the code size.

For example:

$ nios2-stackreport hello_world.elf

Info: (hello_world.elf) 29 KBytes program size (code + initialized data).

Info: 92 KBytes free for stack + heap.

It shows me that the program size is 29KBytes. I have a 128KByte onchip-memory, thus it reported I have 92KBytes for stack and heap.

You can reduce the size if you are sharing the onchip memory via nios2-bsp-editor/linker section.

Or you can just split out the onchip memory to smaller size, ie 2 64KByte memory.