Paint.NET :: Setting Resolution To 300 DPI?

I was asked to design an advert for a magazine by one of my web clients.....

I went and set the size of the advert to A5 (see original settings attached) and completed the advert. I got a call from the magazine company saying that the advert was too "small" and that they needed the image to be at least 300dpi. They said that I needed to "set up the palette to 300dpi" (I think they were referring to Photoshop)

Boy, so I'm faced with having to go and redo the whole job.

My questions are: When I set up the new job in PDN it tells me that the New Size is 2.0Mb, but when I finished the job it had gone down to 295kb - How do you set up the whole job to result in a resolution of 300dpi?What is the relationship between pixels/cm and dpi?How do you set up the job for the right size ie A5 but increase the resolution?I'm loathe to ask this question here but will it be better to go and design this advert in something like Gimp?Designing for a webpage is the exact opposite as for designing for print because with the web you want images to be very small. Here I need the finished advert to be 2Mb is size. At the moment, the finished advert is only 295kb.