Resizing a picture that i really like and i want to make a poster out of it can

[MAC NOOB =(((]

someone teach me how to do it? On photoshop cs version 8.0.
and i want to make it a4 size the whole picture.
i tried to do it but its really blury and i can't seem to make it the exact size i want it to be like for example when i change the width to like 10 cm or somehing
the height will automatically change to something else?? please someone help me thank you very mouch guys

i have attached the picture that i wanted

 

[angelneo]

There's nothing much you can do about the blurriness, unless you manage to find the same image with a much higher resolution.

The proportions are constraint by default, so if you resize the width, the height will be changed accordingly. Resize the image first, then change the canvas size to A4 cropping out the unnecessary part.

If you are looking for the out-of-proportion resizing (not recommended), just unchecked "constraint proportion" when resizing.

 

[SmurfBoxMasta]

or hold down the shift key and grab the bottom right corner and drag it out the size you want. This stops the distorting of the relative dimensions

but yes, if you try to enlarge a low-res pic, it gets blurry fast......you have to start with a much higher res original