Are you referring to the black case with the embossed Mickey parts (ears, pants, feet, etc) that zips closed? If so, I purchased one for my iPhone in August. I don't know what an otterbox is, but here are the interior measurements of the case: 5.25 long x 2.25" - 2.5" wide (width was harder to measure). The exterior dimensions are: 5.75" x 3".
I hope this helps.
ETA: I just googled an otterbox case. I went to their website, and it looks like they sell many different types. I guess it depends on which one you own and how bulky it is. My iPhone definitely has room to slide around in the case....maybe .25" on all sides. Does the otterbox add that much width and length onto the iphone?