A post from the Dia mailing list:

I was asked to deliver some diagrams Dia diagrams I did, in PNG format.

I just would imagine the nightmare it would have been to do this under "other" diagramming tools.

for D in *.dia; do dia -t png -s 1024x $D; done

I just checked them afterwards to do a second pass to save bigger diagrams in a different size.

Thanks for this option, Dia developers!