Mealworms are easy. You need a bin, grain-based bedding, and moisture.
Any shallow bin will do. For grain, I use chicken layer crumbles- non medicated. You could also use chick starter, however I have switched over to layer feed for it's higher calcium content.
For moisture, I use half a potato, and replace as needed.
You will need to sift occasionally, although I usually add more grain until the bin is full. Sifting is not my favorite job.
That's really all there is to it.
As far as size, toads will eat whatever will fit in their mouths, although if in doubt, smaller is better. However mealworms should not be used as a staple diet. They are harder to digest and less nutritious than most other feeder insects.