RAM placement

I’ve been building computers for a while now, and its one of my stronger points in the technical field. But what I’ve always wondered is what slot should each stick of RAM be placed. Now when all sticks of RAM are the same speed and size, its obvious to me it doesn’t matter where there placed, but what if one of the sticks is larger than the other. Should this be placed in the first DIMM slot? The last? and what if on stick is faster than the others, should it be placed first or last? Or does it not matter at all. Ive always put the fastest/biggest first. much appreciated for any help.