The original measuring cup has a capacity of 175ml (whole portion). It is a necessary tool, I understand that without this information you would not be able to use the rice cooker. Until you find a replacement, you can use a measuring cup or glass with notches.

I measured it out with basmati rice. Quite full is almost exactly 150gr.

I've just measured my mug from the rice cooker with water: 166 ml goes in almost to the brim.