Regarding the size, the ND filters come in square 100mm. The graduated filters are longer so you move them up and down without worry.
I have no problems with the Lee filters on the 16-35mm II and 5D. The only problem I have is vignetting below 20mm when I use the polarizer, I have two filter slots on the holder before the CP holder, if I droped this to one filter slot then I could get closer to 16mm before suffering from vignetting.
Solution is to have one filter holder with one slot and the polarizer holder and then then another holder with 3 slots for filters.