PDF pages of different sizes when using continuous view in Acrobat

I am relatively new to Acrobat and am looking for some help.

I wish to create a single PDF document by combining a number of differently formatted files - jpeg, tiff, and pdf - using Acrobat. The files that I am combining have been created by scanning 8.5x11 pages on my scanner. As mentioned, these image files are in various formats. When I scanned them I made sure that the scanning resolution size, source size (ie. paper size), scaling size, and new pixel size was the same irregardless of whether the file was a jpeg, tiff, or pdf.

When I create a pdf from these multiple files (Acrobat 7.0 pro), and view the pages using 'continuous view', the pages are displayed in different sizes. The pages that were originally scanned and saved as pdf are invariably larger in displayed size than those scanned and saved originally in jpeg or tiff. I would have expected all of the pages to be combined and displayed at the same size as my scanning settings were consistent.

Is there a way to get all of the pages to display as the same size when viewing the document in 'continuous view'. I tried using 'crop pages', but I had no luck and suspect that this isn't the way forward.

I would appreciate any insight and suggestions.