Prints not in the catalog can be special ordered but there are basically two types of orders you can make.
Photos shot as digital files can be requested in any of the normal sizes.
Photos shot on film need to be retrieved from the archives, scanned and Photoshopped. So these can only be ordered as a 17X22 size, either as a poster or fine art print.
Photos larger than 17X22 are available but need to be sent out to a service bureau, price to be determined.