Exporting to Print at a Laboratory

I want to create a 16" wide by 20" high print. I have done this successfully in the print module and used print to file . Everything looks ok but the picture looks paler/less vibrant. So I went to the usual export process in the library module. . The aspect ratio is set at 4x5 but lightroom will not export at 16"x20" . It will keep 16" but then changes the other dimension to 12.5" . When exported if I try changing it to 20" high the horizontal dimension moves to 25". I'm not sure what's going on . How can I export correctly at 16@ x 20" ?