The only way to destroy the existing proportions of the image in Lightroom is to crop the image. If your image was 3:2 out of the camera, it will remain 3:2 unless you crop it.
If you crop the image, you can achieve any proportions you want.
Export does not change proportions, it only can change the number of pixels.
If i resize image to say 1920 for the long side but have no other measurement, will Lightroom keep the images in proportion?
Lightroom *always* keeps the images in proportion - the dimensions you specify can be thought of as a "bounding box" - images will be within dimension(s) specified, but may not "fill out" the other dimension.
does lightroom always see the long side as the width?
No - long side may be width or height - so if you have a mix of landscape and portraits, the landscapes will be 1920 wide, whereas the portraits will be 1920 tall.
If you set the long edge to be 1920 pixels, then your landscape images will be exported 1920 pixels wide and your portrait-oriented images will be 1920 pixels tall. But the images will be in what ever proportion they were in prior to the export.