edit: did misread before,
here for the size related Q: i think this isnt really possible unless you know the exact size of the base surface and set the tiling exactly according to a fitting value (must probably calculate a bit) you can also use the manual crop box size values to help define size and or space between the items.
so as an example, if your object is 10cm and the U length 150cm you can set tiling to 15, etc the scatter object will be 10cm in size then.