Here is an example of how well LR can do at resizing. I have used these images as examples previously, but not in this specific context.
Here is an image that I wanted printing at 30"×20", and the pro lab service that I use requires you to resize the image to meet 300 PPI, which for that print size is 9000×6000 pixels or 54 MP. Here is the original full image resized to meet the POTN posting limits.
This is a 100% crop of the original sized image.
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In the process of making the image print ready I cropped the original image a bit so that it was 4082×2698 pixels. I then exported the image from LR, letting LR handle all of the resizing in one go. I shan't post the full 9000×6000 pixel image, but here are a couple of 100% crops from the enlarged image. This is a pixel enlargement factor of 2.2×.
I will admit for full disclosure that I actually exported the files used here specifically for use on POTN, and I used my usual POTN Output Sharpening settings of Screen/High with a PPI setting of 100 (the output resolution setting supposedly affects the level of output sharpening applied, so may be a relevant setting in this situation. The value used makes no other difference to the output).
I think most people will agree that the image seems to stand up well to this level of resampling on export. I can tell you that the print, done on Fuji Crystal Archive Pearl paper, looks absolutely stunning and holds up to nose distance scrutiny. Since the aircraft is done in a pearl finish paint using the same effect in the print paper is really rather magical in effect. Oh and yes it was shot handheld, with a shoulder stock support, at 600mm and 1/160s with the Sigma 150-600 C and a 50D, with the subject moving at over 100mph.