Lightroom does non-destructive editing, so it never changes your originals, and you cannot resize them.
All edits are written to the catalog (which is a database), and are only applied to a copy of the original when you export.
If you want to export at a different ascpect ratio than the original, you have to crop the image to that aspect ratio first.
In the Export dialog, then choose Long edge (or Short edge), and enter the dimension you want to export to.
You may find this article helpful: Lightroom basics
What do you mean "no-destructive editing"? How's changing the image size any different than changing contrast and other parameters? When I try to export to a JPG format I cannot even change the image quality (LR assumes some settings, and judging by the image size as compared to other programs, the quality must be below 100%). LR is a joke.
Non-destructive editing means that no pixels are changed in the original file, so that you can always go back the original, and do addidtional editing, or change your mind and start over, or create a different version of the file (Virtual copy). All the edits will be visible and undoable in the History panel when you return to the file.
Editing in Lightroom works exactly the same way as the Camera Raw plugin in Photoshop,
Lightroom was primarily designed to work with raw files (but will work with jpgs and some other formats too), and raw files cannot be edited the same way as other files. Only a digital camera can save raw files, so edits are applied to a new version of the file - derived from the original, and it will be created when you export. Lightroom works the same way with jpgs as with raw files, which is actually a very good thing. The jpg format uses lossy compression, which means that every time you save a jpg, it will lose quality, and that quality loss will be visible. Repeated saves of a jpg can make the image disintegrate, and you will end up with a very low quality file.
When you edit a jpg in Lightroom, it is never resaved, and you avoid the quality loss.
When I try to export to a JPG format I cannot even change the image quality
Sounds like your original is a jpg, which you are exporting as Original, which will export a copy of the original, unedited file.
Choose jpg instead of original in the export dialog, and you can set the quality on a scale from 0 to 100.
Before you conclude that LR is a joke, I suggest that you spend some time learning how the program works.
I also suggest that you read the article I linked to. There are also much more comprehensive tutorials, like Julieanne Kost's Lightroom videos: Lightroom Tutorials by Julieanne Kost or Adobe TV: Lightroom Classic tutorials | Learn how to use Lightroom Classic CC
The Lightroom Queen is also a great resource: https://www.lightroomqueen.com/
The original picture is in a raw format (DNG) and I want to export it to a JPG. When exporting to a JPG format LR won't let me specify the quality (compression), which most graphical editors support (such as free Preview on Mac). I downloaded a trial version and found the software not very intuitive and limited as it doesn't support basic functions that I expect from such program. Sorry, don't have time to read tutorials, these shouldn't be required for such basic functions.
The original picture is in a raw format (DNG) and I want to export it to a JPG. When exporting to a JPG format LR won't let me specify the quality
You probably have Limit file size to checked. Uncheck it, and the quality setting becomes available.
Sorry, don't have time to read tutorials
Lightroom is very different from most other image editors, so reading tutorials will save you hours of frustration.
At least read the one I linked to first - Lightroom basics - it will only take 10 minutes, and covers the most basic aspects. (I wrote it)
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Sorry, I made the mistake of assuming that you were using Lightroom Classic, but you seem to be using the new Lightroom CC, which I don't use.
I'll get a moderator to move this discussion to the LR CC forum.
I have the same problem but I cannot fix it since I don't have the File > Export option ... The only option I have is the one pictured above by 1211001557. I'm on Mac. Might it be the cause of this? I need to resize my pictures by the width and I cannot find the option. Can you help? thank you