I'm sceptical about any candidate winning "big", at least in terms of popular vote. The country is polarized politically and, let's be honest, both Hillary and Trump are polarizing figures on their own. Right now a narrow PV victory (possibly with Obamaeqsue EV victory) for Clinton is the most likely possibility.
But, again, there's so much time till November. She can screw up big time like Dukakis in 1988.
You are right that we are hyper-polarized right now, but that doesn't exclude the possibility of a landslide. It may minimize the size of one, though. I think a 10pt win by any measure today would be a landslide, especially since Libertarians will take even more points from Trump.
The way I see it is, Trump is going to lose non-white voters by even larger margins than Romney's 17%. I don't believe he will get Romney numbers among whites, either. Not based on how terrible his behavior and image is right now, or the fact that he is now flip flopping on a daily basis. Those two factors, to me, make a close election impossible. If Hillary were to get Obama's vote shares from 2008 (an actual possibility), it would be a >55% win. That's how badly demographics are hurting Republicans.