In general, you should be aware of your limitations depending how far you’ve advanced. Below is a rough estimation of the difficulty of enemies taking into consideration how lethal they are and how easy they are to overcome. Some preparation is needed even for low level encounters; there definitely is a large difference between fighting a skeever and a troll. If you encounter an enemy that is in a higher category than you think you can take on, you might want to head in the other direction.
Consider that the encounters below generally hold true, but can be drastically different if fighting more than 1 enemy at a time.
Low-level encounters
Mid-level encounters
High-level encounters
End-game encounters
Skeevers, mudcrabs, slaughterfish, horkers, sabercats, wolves, bears, bandits, skeletons, small spiders, non-magical draugr, ice wraiths, trolls
Magical draugr, hagravens, witches, falmer, forsworn, larger spiders, atronachs, soldiers, smaller dwemer automatons, rieklings, ash spawn, chaurus
Giants, vampires, mages, dragons, dragon priests, boss humanoids, boss draugr, ash guardians, larger dwemer automatons
Dremora, werewolves, slighted, lurkers, seekers, frost giants, boss vampires