How bad is a "bad" oil spill?
Is there a clear guideline as to what exactly constitutes a bad oil spill?


No, "bad" has no legal definition in any jurisdiction in North America. As a practical matter you can only establish regulations and enforce legal penalties, regulatory, civil or criminal by some sort of numerical, measurement based criteria.


  How much oil must be spilled? 


All material economic activity has some sort of negative tradeoff, all jurisdictions have some level of permissible harm. Each jurisdiction a pipeline passes through will have some level of oil leakage or other types of environmental harm that are permitted without triggering as legal consequence for the operator. However, this does not define "bad" but merely the trigger threshold.

An international pipeline like the Enbridge pipeline that crosses countries, provinces, states etc will be subject to a different set of rules for every jurisdiction and at times combinations of jurisdictions. You'd have to check each individual jurisdiction to find the threshold for each one. 


  Is it dependent on what type of oil is spilled...


Usually, yes. Since the primary harm from oil is mechanical, the viscosity of oil is the prime predictor of the harm it will cause if spilled. Crude oil varies in viscosity from a thin syrup to thick paste. The former can spread fairly far in warm weather while the later usually just piles up at the leak. The former causes more harm per unit of escaped volume than the later. 

Most of the oil moved in the Enbridge is dilbit a diluted form of bitumen/asphalt from the tar sands. Bitumen/asphalt is arguably a solid so it has to be diluted with something else to make it a pumpable liquid. Depending on the viscosity chosen, the spread and mechanical risk will vary as well.

The diluent itself is going to be something volatile which could in principle constitute a greater toxicity danger than simple crude oil. Naphtha has been used in the past which is the most volatile fraction of any crude oil. Depending on the actual ratio, spilled dilbit will present a greater or lessor danger than a comparable (usually light) crude. 


  and in what area,


Quite definitely.  As noted above, the oil does not spread to any significant degree without water transport. A pipeline crossing a body of water or running near one will require more rigorous construction and closer monitoring than one that does not. Oil pipelines crossing desert or arid regions cause much less harm be unit of leaked volume than do pipelines in swamps or under body of water.


  populated by what species?


Usually, but again, this varies considerably depending on jurisdiction.