Would suggest you to make sure that the printer drivers should be updated.
Also, try to print to file from Lightroom and check if you are able to get the exact paper size through that.
Printer driver is current. When, for instance, I choose 4 X 6 profile and then in the P600 print/printer dialogue enter 4X6 paper size, the print preview (and print itself) is only a partial image in the corner of the paper. Same happens trying 5 X 7. The only size that works is 8 X10.
Thanks for the update.
Did you try to print to file through Lightroom to check the same?
Does the same happen when you print through some another program apart from Lightroom?