You just need to specify an explicit ImageSize . This can be set in the initial GraphPlot or changed using Show as follows:
gp = GraphPlot[ {3 -> 4, 3 -> 5, {3 -> 6, "edge 3->6"}, 4 -> 6, 5 -> 6}, EdgeLabeling -> Automatic ] Export["graphplot.html", Show[gp, ImageSize -> 800], "HTML"]
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