To determine what size I want them to be, I will print the design on regular paper on fast draft (to save ink) and lay on the shirt. It is easier to see this way to see if the design should be bigger/smaller. Then I will repeat this process until I am happy with the size.
As far as placing on the shirt. I normally have the person put the shirt on and hold the design to their chest. Then I roughly measure how far down from the color I want the design. Then when I am placing it on the shirt I know how far down I want it. Then as far as centering right to left, I normally just either eyeball or measure from the sleeve lines to make sure it is centered.
I hope this helps gets you started.