I recently switched to using LO (v7.6) on Win10 and am not able to print out a #10 envelope on my Canon printer MX870 with the correct orientation and font size – despite making sure I have the correct selection of page size and orientation.
[Note – the two sets of lines were results of two attempts].
It seems that LO stubbornly sticks with portrait orientation and scales the font size to fit its conceived paper width.