Canvas Size + Resolution= File Size??

Can someone please explain to me about file sizes and image dimensions. I have to put together a collage which
is 165mm high by 258mm wide. If I click file new and choose default resolution (72) then the file size is ok, if I type in 300 dpi the image becomes huge. If i chosse 300 dpi then the images I was going to use seem tiny on such a vast workspace - Im confused why does the canvas become so large when changing resolution?:confused:

Set the resolution first, then set the size. A change in the resolution changes the dimensions in proportion. After you change the res. you can revert the dimensions to what it was.

Thanks

but I have done that and my image size comes out at 146 MB!!!

seems a bit big

when changing the dpi you changes the dots per inch (dooh :P) .. therefore, when youre collage is set to 72 dpi its becomes way smaller on 300 dpi..

So if you want to have your image in 300 dpi, its either going to be small or very 'blurred'.. But if you insist on making it 300 dpi go to image -> image size and change the dpi (make sure the resample option is checked), and that should do it.. as i recall, i cant check it now, but if im wrong someone will correct me ;)

300 dpi is used for high quality stuff.. if youre just going to print on a printer 150 dpi is more enough..