The size of the media dicates the size of the photo that the printer will print. So a ribbon with a siz inch width an a 9 inch pane size will only print to a maximum of 6x9. Some printers will however also allow images of 6x8 and 6x4 to be printed as well.
When printing smaller sizes the part of the sheet which is unused is "wasted". So regardless of whether you print 6x8 or 6x9 images you will still use the same amount of media.
When printing 2x6x4 images on 6x9 media then you will be paying the same price for 2x6x4 images as one 6x8 or 6x9 would cost you. You many find it a lot more economical to use the correct 6x4 media as you only pay for what you use.
Most printers do not have the facility to print 2x6x4's from a 6x9 image and it can be driver dependant. The Click System, HiTi 510S, HiTi510K and Citizen CW-01 all support 2x6x4's from a 6x9 but only on a PC platform as it is driver and printer specific
Likewise a printer that uses 12z8 media should be able to print a 10x8 on the same media however the reverse is not true of course. Check that you printer will support this as not all printers will.