What is the pdf veiwer size limit?

Not a 'hard' limit, no. But, what you describe is an app crash due to memory issues. Try resetting the iPhone before opening the document. To reset the iPhone, press and hold the home and sleep/wake buttons until you see the Apple logo (ignore the slide to power off prompt). Alternatively, there are memory freeing apps in the app store.

30 MB.
In using the iPhone's iDisk app to access and view a 68 MB PDF file, I get an error message on the iPhone saying that since it's greater than the 30 MB limit for PDF files, the file cannot be viewed.

PDF Expert in the app store has no limit to pdf size.

http://itunes.apple.com/us/app/pdf-expert-professional-pdf-documents/id323133888 ?mt=8

GoodReader version 2.4 works fine with the document in question.

The Good Reader app has no limit for PDF size:
http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=306277111&mt=8
http://goodiware.com/goodreader.html

A very good question!! I loaded Docs To Go, Good Reader, and they changed nothing at all.
I was having issues opening a 9.95MB PDF file directly from a corporate website using Safari. Smaller documents (1.2MB) loaded no problem.

The document loads OK on the pc, but not on the iPhone. Also refuses to open using Docs To Go or Good Reader regardless if the file is in a remote folder in a cloud or in a subfolder directly on the iPhone.

A couple people here claim to have opened much larger files than that. I'd be interested in a confirmation that you can open this one: http://www.stm.info/info/reseau2009.pdf

My thanks in advance