There is no particular size limit that I'm aware of for a template, form or attached file. However, if you open a large form or a form with a large attachment, you will experience performance issues. Performance has been discussed in these forums before and you should be able to locate it via a Google search.
As for InfoPath books, check here for a list of them (though I'm afraid none of these will be documentation-type books):
http://www.amazon.com/gp/redirect.html?link_code=ur2&tag=braintrove-20&camp=1789
&creative=9325&location=/gp/search%3F%26index=books%26keywords=infopath%26_encod
ing=UTF8

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Greg Collins [Microsoft MVP]
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