Does anybody know the best way to export raw files from Adobe Lightroom so that they are able to be uploaded on UER?

Export them as a JPEG?

File > Export

Then choose output folder.
then play with 'file settings'

(here are mine for my posts)

click to view

Then click export to create JEPG.

Does resizing it to those dimensions lower the file size from ~20mb to 100k smaller?

It will compress, and convert your photo from a RAW to a JPEG.
In the attached image I showed you, you can set a file size limit to what ever you'd like, I set mine to be no more than 400kb. less is better for bandwidth reasons, but less is also less image quality. Personally I think 100kb is much to small. The quick gallery creator isn't restricted to the 100kb max file size. Changing height and width also makes file sizes smaller.

Posted by FoMoCo It will compress, and convert your photo from a RAW to a JPEG. In the attached image I showed you, you can set a file size limit to what ever you'd like, I set mine to be no more than 400kb. less is better for bandwidth reasons, but less is also less image quality. Personally I think 100kb is much to small. The quick gallery creator isn't restricted to the 100kb max file size. Changing height and width also makes file sizes smaller.

I know it will compress and convert raw to JPEG. I have posted many photos before. Also, my files are unable to export to 100k. The AvGalleryCreator v1.2 only allows files of 100k or smaller it says(although I could be mistaken on that fact). What I do is change the color space to adobe rgb(1998) and it works right away. I was unaware about changing the size as a better option. I will try it, that you very much!

Just host it on Flickr. I started doing this and the images turn out MUCH better than using the UER uploader here where compression absolutely ruins the photos.

Are you getting server or script error messages while trying to make new threads as well? I've been having this problem for a month or two and haven't been able to post any of my new shots