volume versus ounces? what should I be using to measure my food?

I am almost five months out - I just recently read that I shouldnt be weighing my food, but rather using cups to measure the volume...is that the case? does it matter if its liquid, dense protein etc? totally confused about this. 

The bottom line is that your stomach does not have a scale, so volume is the best measure.  On the other hand, volume is not always a convenient measure, like when you are measuring meat.  For meat, I use the scale, for anything that would fit into a cup, I use a cup.

thank you so much Elina!

So at 5 months how much should one be eating at a sitting if using a cup to measure like cereal for breakfast?