I have several collages composed in Lightroom, and would like to print them at 21x15. Can you please recommend appropriate Lightroom settings for exporting them? I would like to avoid any crop or black/white strips on printed photos, once I give photos to the print company.
With the clarification in the comments and I think the edited question I think the real answer is you should not use the Lightroom Print option for sending images to a print company, but should use Export.
Generally speaking you should crop the image to the right aspect ratio first, then export without resizing in the format (JPG, TIFF) and color space (sRGB, Adobe RGB, etc.) that the print company wants. If they do not specify use JPG and sRGB.
It is usually best not to resize, but let the print company do so, unless they specifically require that you resize.
There is also a sharpen option for the export, where you can select the amount of sharpening and type of planned media. Generally, if you have modestly sharpened, you can do a medium or low amount of sharpening here for the correct media; if you have strongly sharpened your image in lightroom you may want to turn this off.
Using the Print option with JPG as output is possible, but it goes through extra steps and involves borders and such you normally do not want when sending to a custom lab to produce prints, and you risk getting something like borders twice - once from the module and once from the lab.