So, for the first one Export it cropped to 6.5 x 9 - and in the Export Dialog box there is a capability to rename the file for exported output - which would allow you to add something like the crop or intention to the base name of the file. e.g. for _IMG0846.nef - setup the output filename to _IMG0846-65x9 for 11x14.jpg and output it at, say 300 or 360 dpi. So, it has a unique name and purpose in your output folder.
For best results, now, re-crop the picture in LR - just open the crop [R] dialog again, reframe the image to the new ratio, 12x16 and Export again - with an adjustment to the filename. e.g. _IMG0846-12x16 for 20x24.jpg
So, now you have two clearly identified image files in your output area and you can provide those to whomever is printing for you, etc. Discard them when you're done since you always have the original to return to to re-frame again as necessary.
This way you get output sharpening and resolution (in pixels) optimized for your output print size. Maybe not super important for smallish prints-but more important for larger ones. Output at 100 quality.
In addition to jimcamel advice (which I agree with), you might (optionally) want to use virtual copies to save multiple crops in your catalog, so you can always come back to the specific cropped photo if you need it again in the future. This takes up less disk space than saving the exports.
By the way, I noticed that jimcamel does not use a decimal point (or period) in the file name where the file size is 6.5, which again I think is good advice, using a period here can cause problems with some programs.