Blank Canvas

Hi i am deperate to put a stroke around my canvas i have created and am totally stuck.

I want the stroke to be smaller than the canvas size by 25mm all the way round . i can do it using the retangular tool but i have to guess to get it equal.

There must be an easier way guys

I have windows and CS3 so this is my experience:
#1 Highlight your layer in the pallet window.
#2 Go to Select>All
#3 Edit>Stroke
#4 Edit Color and Width.

1. Press CTRL+R to display the Rulers
2. Right click on any ruler to change the units of measurement to millimeters
3. Click in the horizontal ruler and drag and a guide will appear; position it 25mm from the bottom and repeat for the top.
4. Click and drag in the left side ruler and position guides 25 mm from both edges.
5. Use the Rectangular Marquee to select the areas outlined by the guides then use the Edit > Stroke command to select the stroke weight, width, and position relative to the guides.

Note that you cannot move guides after they are placed(you can delete them but you can't drag them after releasing the mouse) so be careful to line them up correctly.

Also, if the rulers don't show enough precision, Zoom in and the units of measurements will expand.

Brilliant but how do you deleat the guides once you have finished? Sorry just worked it out. Thank you for your help

View > Clear Guides.