...I was taught that if I need to print one size...
Sorry, I don't understand what you mean by this.
Any image can be printed at any size.
The PRINTED size (In mm, inches, miles, kilometers, whatever) is controlled by the RESOLUTION.
Resolution has 2 hats...
Hat #1:
The resolution of an image is no more than a label, a tag, a piece of information....written into the image file.
Your 'printer' (Of the Plastic kind) reads this piece of information and uses it to calculate how BIG your PRINTED image should be.
For eg.
An image 300x300 @ 300ppi would give you a PRINT size of 1" x 1".
An image 600x600 @ 300ppi would give you a PRINT size of 2" x 2".
An image 300x300 @ 150ppi would give you a PRINT size of 2" x 2".
An image 600x600 @ 150ppi would give you a PRINT size of 4" x 4".
Note: INCREASING the resolution DECREASES the PRINT size.
The 'Pixel Dimensions' remain CONSTANT regardless of the resolution used.
Hat #2:
The number of pixels in an image (Its pixel dimensions) can be changed.
This process is called 'scaling' or 're-sampling'.
The 'resolution' can be used to re-scale the image by adding OR removing pixels to or from the image.
This is not usually a good idea either way as adding or removing pixels is destructive and cannot be undone.
This can be used to keep the PRINT size CONSTANT by changing the number of pixels in the image.
For eg.
Take an image 300x300px @300ppi.
Its PRINT size would be = 1" x 1".
Keeping the PRINT size CONSTANT we could change the 'resolution' to say 150ppi.
The 'scaled' image would then be...
150x150 @150ppi = 1" x 1"
In other words the image has got smaller, its pixel dimensions are half that of before....which equates to a quarter of the total number of pixels....this is why it is not desirable to re-scale images.
Note: DECREASING the resolution DECREASES the pixel dimensions but keeps the PRINT size CONSTANT.
Keeping the resolution CONSTANT means:
1. You can re-scale the pixel dimensions by changing the PRINT size.
2. You can change the PRINT size by changing the pixel dimensions.
What you are doing at the moment is creating a NEW document that is already set correctly in terms of 'resolution' and 'pixel dimensions' to give you a PRINT size of 18" x 24".
If you change the resolution later WITHOUT re-scaling then you can have ANY size print you want.....thats Hat #1 BTW.
And yes, if you need your resolution to be 300ppi your settings are correct....they give you an image of 5400x7200px which is VERY big.....more than enough to work with.
(Its always best to have too many than too few)
Regards.
MrTom.