I only came across the render dimensions tab the other week after someone advised me too. I set it to 300 as previously i was rendering at 72 dpi.
Then I go to the render setting (as I always used before)  and type in something like 2000 x 2020(it automatically puts the second figure in). All I want is to display what I have on my preview screen and print it out at 300 dpi resolution in A5, A4 and A3.

Normally I would just put in 1000 and it would automatically render my image as in the preview screen. But no I have changed this amount to 300 in "render dimensions" it doesn't always render the whole picture. Do I have to adjust the render dimensions 1000 x???? select 300 dpi, and then go to render settings and put a diff amount in or the same ? Very confusing I take it I was rendering at 72 dpi before? There does not seem to be any quality diff between 72 and 300 apart from seems to take longer to render.