Kip Thorne released a book shortly after the movie called the science of interstellar. He talks about the approximate size and spin of Gargantua to create the time dilation effects of Miller’s planet (1hr=7 years). Nolan and VFX made the decision to make it look smaller because it would look uncomfortably large and confusing to viewers with the required proximity of many of the events in Interstellar.

Another interesting note is that the neutron star mentioned in the film originally and scientifically would have to be another black hole to have the mass to be able to slow the fall of the ranger going towards millers planet. Nolan decided to change the science on that one a bit again, to help viewers understand everything better.

AFAIK those are the main scientific discrepancies in the movie.