Commercial graders are at least $150 each, and the one size fits all grader holder, will run close to $300.
Graders come in many different sizes, (18's, 19's, 23's, etc) and the numbers mean the grader size in 64ths of an inch distance between bars.
An easy way to make a poor man's grader is to find hardware cloth (the metal screen that minnow traps are made out of) in two different sizes, two square and three square (three square is hard to find). These are the number of squares per inch.
The two square will hold fairly large minnows, and the three square will obviously hold smaller minnows.
For each grader, take some 1x6 boards, and make a two foot x two foot box. Then fasten a piece of either size screen on the bottom. It's best to make two boxes, each with a different screen.
When grading your bait, stack the two boxes one on top of the other, in about a foot of water (with the box and larger screen on the top.
This way, the large minnows will stay on the top box, and the smaller in the second (lower) one.
All of the undesireable size will swim through the bottom of the lower one, and return to the pond.
When putting your bait in the top box, snap the surface of the water with your finger and this will drive the minnows down, trying to get away from the sound.
This will speed up your grading process.