Response from the site administrator
Zoner, 07.01.2011
It's not possible to crop to a physical size -- one in inches, etc. -- because the physical size that you get from, say 10000 times 10000 dots depends on how many dots will be printed per inch. If the lab will use, say, 300 dots per inch, this gives a much different size than 600 dots per inch, for the exact same crop!
But it's true that it's still useful to crop to a ratio, and definitely to things like 6x4, 7x5, etc. We even do have some presets for ratio crops like that. These are called "Fixed Aspect." Check the Editor toolbar when cropping, and drop down the box that says "Free Ratio." For example, the 3:2 Fixed Aspect crop gives a 6:4 crop, as 6:4 and 3:2 are the same thing.
Advice on which ratios are most lacking in the list is definitely appreciated.