Although there are many types of cannabis seeds available (regular, autoflowering, feminized, etc.), it is impossible to tell them apart by simply looking at them. To be hundred percent sure that the seeds you ordered are authentic, always buy from a trusted and respected seedbank.
The Seeds can come in many sizes and shapes (i.e. indica strains seeds tend to be bigger than the sativa ones), but usually they are not bigger than the head of a matchstick. You just have to remember - the size of the seeds has nothing to do with the size of the plant that it will produce, or how long it will last.