Flat laces are matter of style. But I am letting you know that 2.5mm for ROUNDED ones are about the thickest you want to do.
I actually have all 2, 2.5, 3mm ones. 3mm makes my bals look like athletic shoes. 2mm is the one AE sends you with their bals. 2.5mm is the one that is aftermarket AE laces (ones with the color). Neumok laces are 2.5mm, as an example as well.
Be careful with some Ebay listings. I bought a whole bunch that were listed as 2.5mm, but ended up being 3mm when I received them.