Daniel Prince wrote:
I like to use ribbed plastic screw anchors to hold screws in drywall
for light duty use. My problem is that they never come with ANY
instructions so I always have to guess what size of drill to use and
what size of screw anchor to use.

The plastic screw anchors I have come in four sizes, each a
different color. From smallest to largest they are gray, red,
green, and yellow. I use mostly drywall screws because they are
inexpensive and easy to drive with a cordless drill. Drywall screws
are number six when they are two inches long or shorter.
Two-and-a-half inch, and up, are number eight.

I have been using the red plastic screw anchors with the number six
screws and the green ones with number eight screws. Are those the
correct sizes?

What sizes of drill bits should I use for each size of plastic screw
anchor? What size of plastic screw anchor should I use with each
size of screw?

Those plastic anchors are not designed for drywall screws. Drywall
screws suck in shear and have thinner shafts than other screws. The
size of the drill bit should be the same as the smallest diameter of
the anchor (diameter without ribs or fins).

R