Agent orange is normally contaminated with other chemicals that cause mutations. Again, you run the danger of mutating the mosquitos into giant Hollywood-style mosquitos.
Unless you're proposing that several interactive mutations would occur all at once, I'd say go for it. From what I recall of general insect biology, the suckers'd be so big they wouldn't even be able to stand up, and their respiratory systems aren't designed for very large sizes.
yes, and extinct for many millions of years :) They were probably killed off by fish and other more mobile critters.
Sea creatures can grow larger for the same basic shape than do land creatures because the water helps support their weight.