It all depends on the strength and stats of your server machine and on what operations people are going to do. It also depends on what the exact makeup of the file is (for example, if you have a pivot-cache in the file, less of it will be loaded into memory).
I suggest you slowly up the file size and see how it works for you, or try to give more information about your deployment (what machines, how many workbooks, what they contain etc) and I may be able to point you at a better way of figuring out a right number.
Shahar Prish
Blog: http://blogs.msdn.com/CumGranoSalis
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