The Indominus rex was 50 feet long, or 15.24 m.
According to Wikipedia T. rex are 12.3-12.8 m and 4.5-18.5 tonnes. Taking average of both we get 12.55 m and 11.5 tonnes.
(15.24/12.55)^3*11.5 = 20.59 tonnes, Class 25
However, most palaeontologists usually estimate the weight of a T. rex to be around 8.5 tonnes instead, and the average 11.5 tonnes is dragged up by a high-end estimation of 18.5 tonnes, in which the authors themselves questioned the validity of their own calculations. So I'll use 8.5 tonnes instead.
(15.24/12.55)^3*8.5 = 15.22 tonnes, still Class 25