Arduino Nano periodic reset under program control?
Why do you want to reset you board?  The reason affects the answer.

Do you want to reset the board because your program crashes after n hours?  If so its probably better to fix the memory leak that you have in the code.

Is it because some global variables need to resetting every now and again?  If so could you place them in a class and store a pointer to them as your only global.  When you want to 'reset' the board delete the instance of the class and recreate it.

What happens if the software crashes?  Have you got it configured to restart on a crash?  If so could you just crash the code deliberately?

Because you have size constraints then it may be worth a bit of lateral thinking to see what the code can do for you.