Do you paragraph a proof? When writing out a proof of moderate length, i.e. a proof taking less than or equal to 5 A4 papers and with normal spacing (please avoid asking the criterion for "normal"), do you tend to paragraph it or not? Why and Why not? Is there a style guide on such question? 

Proofs should follow the same rules as any other kind of writing.  If the text naturally forms paragraphs then that's how it should be written.  Now, usually if you have five pages of text forming a single paragraph that would be poor style, but I suppose it's possible that you could have a five-page-long paragraph if, say, one of the sentences of that paragraph was a several-page-long equation that couldn't be broken up.
