Does anybody know where is Lightroom 3 I can set the image size and canvas size for printing??
You just need to forget this convoluted way to work.
In Lightroom, it's more akin to enlarge a negative with your enlarger :
- You crop your negative with the Develop module / Crop tool as the marger,
- You choose your paper size in Print module / Page setup (NB you still can output a jpeg) as the paper box.
You can also choose the output resolution if you need to (if you don't check the box it will be the same pixels as original which not that bad a choice).
You still a got a bit more possibilities in terms of layouts.
Alternatively, you can specify a pixel size in the print module, and some external plugins (LR/Mogrify eg) can add margins.