People claim the JP3 Rex was a sub-adult becase it was small but you won't find any source that actually says that. The Jurassic Park 3 script explicitly says it whas a full grown Bull Tyranosaurus. Look it up:
http://www.imsdb.com/scripts/Jurassic-Park-III.html
Now, several licensed material for Jurassic World claim the Tyrannosaurus was 40ft long and in the Stan Winston School of Character Arts it says the Rexy/Roberta animatronic from Jurassic Park is nearly 40ft long :
https://www.stanwinstonschool.com/blog/jurassic-park-t-rex-sculpting-a-full-size-dinosaur
http://islanublar.jurassicworld.com/dinosaurs/tyrannosaurus-rex/
http://www.jurassicworld.com/intel/dinosaur/tyrannosaurus-rex
This is a simple size comparison I made for the JP3 Rex, the JP3 Rex upscaled to 40ft long, and the JP3 Spinosaurus:
https://steamuserimages-a.akamaihd....984/A4E97C537996A591A8BD82D8D2A159B871CC42D3/
Even compared to a 40ft long Rex, the Spinosaurus is considerably bigger. Compare the size of their skulls, then go and watch the vídeo where they fight. There's a moment when the Spinosaurus turns around to face off the Rex. At one point, their faces are in front of each other and so close their snouts clip through each other. The Spino's skull actually seems to be shorter then the Rex's, when it should be considerably longer. That gives an idea of how undersized it is. Some claim it was big and powerful in JP3 because it was a hybrid, but the documents that say it went through genetic tampering never say what traits were changed and how. Until that happens, the JP3 Spinosaurus is the reference Spinosaurus for the franchise. It clearly is bigger then even the largest Rex in the franchise, as it was intended to be in JP3 in the first place.