Exporting at an exact pixel size in Lightroom is not always possible but you can get close.
In LR, setting the "Custom Aspect" and Exporting as a specific pixel size are two separate functions.
In Export- the pixel size options mostly indicate a 'Maximum' value where specified, ie, the image is re-sized to fit "within" the limits stated in the re-size panel.
For 1300px X 2000px Set the Custom Ratio to 13 x 20 and Export Resize to 1300 x 2000px. (Edit- I used 20 x 13 for my sample) )
My Export-Resize to 1300 x 2000px
For 2697px X 1518px Set the Custom Ratio to 2697 x 1518- (or 1 : 1.77785102175346 if you want it as a ratio!)
My Export image- is only one pixel row different!
wobertc gives a good review on HOW to do it.
The missing question is: when you try this, how do your results differ from what you desire?
Thanks for your answer. The way you describe is exactly the way I do it myself. However, I noticed when I enter the custom aspect 13x20 in the crop tool, the aspect shown in the dialogue is 3,3x5,10 which is not 13x20. Is there some way I can get rid of the 3,3x5,10?
ManiacJoe: What I get when exporting my image is a 1294x2000 image.
This VERY annoying change of the user input aspect ratio had been a BUG and/or POOR programming decision in Lightroom for a LONG time. Adobe must think it is a feature as they refuse to fix it. The program rounds the aspect to one near (and some not so near as in your example) an existing aspect ratio in the dialog history.
What I get when exporting my image is a 1294x2000 image
And this follows my other suggestion that the image is re-sized in export to fit "within" the limits stated- not an exact pixel size.
1294 is only 6 pixels 'within' your export maximum of 1300.
Only Photoshop and other pixel editors can be made to crop an image to an exact pixel dimension which, when saved, retains that exact same dimension.
You are definitely correct about the ratio. I must have punched something wrong in my calculator. Its still a 1/2 percent off when it doesn't have to be. I still think Lightroom should leave the value the user puts into the aspect values as it would save a lot of confusion and recalculating the aspect is totally unnecessary.
You being able to see the exact 13x20 custom ratio is very strange indeed when I can't. I use the latest version of Lightroom CC, is there a way to clear the custom ratio history and "try again"?
A possible answer to my 13x20 custom ratio!
If you enter any values in the custom ratio dialog box that are greater than 20, then the ratio is reduced to smaller values!
(I seem to recall reading this somewhere but cannot remember where)
So my 13x20 shows as 13x20
my 19.5 x 20.5 is converted to 1.95 x 2.05
I do not know how - "clear the custom ratio history and "try again"? They could be stored in the preferences file- so there will be a Lightroom guru that knows the answer to this. (edit the preference file?)
I just cleared out my old 3,3x5,10 ratio and entered a new 13x20 which solved my problem...
Can you enlighten me? How did you "Clear out the old ratio" ?