I'm planning to do the same shortly, what length of screw did you end up using?
Are the lengths the same for the front and rear doors?
I bought 3/4" length, but sometimes they were too long. After I installed them on the passenger side door, the handle would not open properly because in this particular instance, the screw somehow restricted the movement of the handle.
I ended up cutting them off just enough so when the nut and lock washer on the inside of he door were tightened down, only a few threads were showing.
I seemed to be always cutting off about 1/4", so 1/2" length may be best. It wasn't always EXACTLY 1/4" and the amount I had to cut off wasn't always the same.
It does depend on other things.
If the plastic hole is damaged - widened more than it is supposed to - then you may need a washer on the outside. The problem with a washer is that if its diameter is too big it will straddle the indentations where the hole is and not fully sit flush with the hole. The next smaller diameter for a washer may be too small, depending on how much the hole has been enlarged if it was damaged.
In a few instances, I needed to get some fender washers and cut a section out with a small hacksaw - think of slicing a middle section of a cake, so the piece would be wide enough to just allow it to fit flush with the hole. You can leave the ends rounded if you want.
I found this was the best solution for a damaged hole, but it took time to cut out and file the home made cut out washer.
I any case, for me, each hole seemed to be different and a one-size-fits-all solution did not work. If you buy 3/4" length and don't mind hacking away, then you can't go wrong.
Otherwise buy 1/2" length and if they are too short in some instances, go back to the store and buy the number of 3/4" length you need.