For some reason I feel like I have more on my plate if it is square. I bought some plain white 8x8's. Pile on the veggies and I feel like I'm having a lot more food. Not sure if tricking myself is good for 'moderation' but its working for me right now.
I dont have square plates but I am using one of my dishes from my best set. It is smaller than my reg plates and a pretty shade of purple. I know understand what you mean about feeling like you get more food. Glad you found something that will work for you.