When I first looked at the projection of Grimm Race, I thought if I had been seeing bipedal monsters or not.
Because the Grimm Race looked eerily similar to that off Bipedal monsters first it was theorized that Grimm race is likely to evolve from the monster race but after extensive research of thousands of years, it is found that Grimm Race is completely different that of monster race.
There may be many similarities between them and Dark energy of Grimm race affect monsters both physically and mentally but still, monster race and Grimm Race is different but they are still called monster because how cruel and Ruthless they are.
The Grimm race monsters may look like Bipedal monsters but they are intelligent as humans as using a different type of artifact like us.
There is no fixed size of them, they came in every size and many of them can increase their size depending
As for their power level, it is different from us.