There is a size indication on the product page of each plant you select.
For Nepenthes, this is generally indicated by leaf span across the plant
Extra small (XS) - <2" leafspan
Small (S) - 2"-3" leafspan
Medium (M) - 3"-6" leafspan
Large (L) - 6"-12" leafspan
Extra Large (XL) - >12" leafspan
For Sarracenia, the sizing guide is as follows:
Seedling - Young plants at least a couple years away from flowering size
Small (S) - Single point division with roots. Likely will flower in 1-2 seasons.
Medium (M) - single point division with roots. Will flower this season or next.
Large (L) - Flowering sized division with roots. These rhizomes are quite chunky, some with multiple points.
Extra large (XL) - Specimen sized plant with very large rhizome and multiple growing points.
For other genera please read the description since the species to species variability is so high that it's best for us to describe the size for each listing.