They would be smaller today because of the cool climate, not because of oxygen levels. Whoever told you that dinosaurs were large because of high oxygen level does not know what he/she is talking about. During the age of dinosaurs, the climate was very warm because carbon dioxide (a greenhouse gas) levels were high in the atmosphere, and the warm climate was responsible for their large size. Dinosaurs are known as giganto-therms to physiologists because they rely on their size (and corresponding small body surface) to maintain a relatively constant and high body temperature even though they do not generate their own body heat. Physiologists point out that if dinosaurs were endotherms (animals that generate their own body heat), they would cook themselves from the inside out. A high level of atmospheric oxygen would have lowered the temperature, making it impossible for dinosaurs to get large, and a high level of atmospheric oxygen is not needed if an animal does not burn energy to stay warm. Reptiles and amphibians only use about 1/20 as much oxygen as mammals and birds. High levels of oxygen would have done them no good since they get all the oxygen they need already.
Dinosaurs are unlikely to be endotherms also because they have growth rings in their long bones. Further, when the climate cooled drastically (about as cool as it is today) at the end of the Jurassic, lots of large dinsoaurs, including many sauropods, the theropod Allosaurus and the ornithischian dinosaur Stegosaurus, became extinct. The smallest known dinosaurs, such as Compsognathus and Sinosauropteryx, also happened to live during the late Jurassic and early Cretaceous. Therefore if dinosaurs were still alive, they would have shrunk to the size they were during the late Jurassic to early Cretaceous. Dinosaurs only evolved large size again towards the end of the Cretaceous, when the climate have warmed back up to levels seen in the middle Jurassic.