The way I have heard it, is that Dinosaurs (as reptiles) grow throughout their entire lives. And before the flood, the atmosphere had like 2x the oxygen, and science has shown that most extremely old fossils/bugs that have been found in sap had 2x the oxygen and lived alot longer. (more than 2x longer) Therefore, dinosaurs used to live really long, and therefore grew really big.

There have been tons and tons of finds of smaller dinosaurs, but most of the time they're put to the side for the big ones. So, some dinosaurs died in the flood, and others lived, but could not grow to be as large, in which case became similiar/the same as the reptiles we see today.

Also, reptiles as large as some prehistoric dinosaurs have been found in more recent days too, there was a dinosaur pulled out of the ocean and they have pictures of it, and a video, and it was still breathing, but had been injured or something and the people on the boat through it back out and kept some samples... will try to find article.

EDIT: If you want to bring Noah into this... he probably would've taken younger (~ smaller) animals on the ark.

EDIT: Found article: http://www.weirdasianews.com/2007/01/24/first-living-dinosaur-found-in-japan/

EDIT: http://www.creationinstruction.org/index.php