URGENT Need some photoshop help!

[FeFiDoh]

Im placing an advert in local magazine but it has to be 45mm wide by 64mm deep and 300DPi. How do i make an image 300DPi? If i goto the resolution thing it does it as pixels/inch pixels/cm and it makes the image HUGE! Need to get this sorted asap as its gotta be sent off to them today and the one i did is no good

 

[Scam]

This is probably better off in the Graphics forum, but as i've strayed in here i'll help out.

The image will be huge on your monitor. Think about it, you normally work at 72dpi - change that to 300dpi and your putting 4 times as many pixels where there was only one before.