Cannabis seeds vary in size. Some are as big as soybeans and some are the size of a mustard seed, but usually they are no bigger than a peeled sunflower seed. But these differences do not help in discerning between particular varieties. For example, you can’t tell the ones which progress to flowering stage automatically from the seeds which produce only female marijuana plants.
Sometimes indica seeds are bigger; however, size is not a viable indicator of health or the potential size of a grown plant. If you want to grow seeds, the best course of action is to get them from a legitimate seed bank.