The PNG exporting exponent writes at a screen resolution (~ 75-100 dpi) that will look pretty small at a print resolution (300 dpi). You can have an image scaling tool that looks pretty short, but is likely to be used, or you can choose a vector format that is more suitable for printing than a raster format such as PNG or GIF.
ArgoUML supports both SVG and EPS, which is probably the best choice depending on what your editor / document processor accepts.
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