I hope somenoe can help....

Its not a very big problem but I need to figure it out for my Art corsework thingy.
Anyway, I was just wondering how I can get an A4 size blank image thingy.... I know I ould measure it ant use the 'cm' but when I do that when I go to print it its never that size (its bigger). I use PSCS2.
So how can I get the images to actually print in the 'cm' size that I set it as?

Sorry its a bit of a stupid/odd question....