2 buckets. 1 with a bunch of 1/2" holes drilled in the bottom.
Put ALL the dubia in the bucket with the holes, while holding it above the other bucket (or plastic bin). Shake until all the little guys fall through the holes. Anything smaller than 1/2" will fall through.
Drill bigger or larger holes, depending on the size you'd like to sort. You can even stack multiple buckets with descending hole sizes to sort multiple sizes.