The coils in my atic keep freezing up - I can not remember them being cleaned ever - so how do I cleam them. Clorex spray to kill mold-then spray with clean water????

This weekend I am replacing my supply trunk line under mY house. I have heard that it should be "stepped down" every 10 feet. I have 40 feet of supply. In the first 10 feet or so 6 runs will come off of it.I have a total of 11 runs. I will be starting out at 24x8. Should I step that down to 20x8 for ten feet and then 16x8 for the remainder of the trunk line, or go from 24x8 for 10feet then 20x8 for 10 feet then 16x8 for 10 feet and then 12x8 for 10 feet, or go from 24x8 for 10 feet and then 16x8 for the rest? After the initial 10 feet I will have another 5 runs. One in each bedroom(3), and bathroom(2). It is a 2.5 ton heat pump with a 3 ton air handler. 1251sqft.