Hello Ruth,
if you're able you need to size your print in Photoshop to the size you want to print.
Depending on your camera the size will differ from other cameras.
However, as a pretty good guess your photos will have a 3:2 ratio
You can change this several ways, cropping mainly to the 8x6 which is a 4:3 ratio.
This would mean you will lose some of your photo unless you're able to increase the canvas size and then use content aware fill, but that's another lesson.
Simply changing the size from the 'edit-file size' menu option won't work very well as if you did resize to a 4:3 ratio without cropping you will end up with a very distorted photo.
So if you want the 8x6 size photo you need to consider this when taking a photo by giving some extra room around the subject to allow for the crop.
Once cropped even without resizing and or changing the resolution, the fact you now have a 8x6 (4:3 ratio) will allow the printer options, if available, to print at the 8x6 size you want.
The reason your printer options are jumping to a size you don't want are many fold.
mainly as mentioned the ratio is wrong, the ppi of the photo is too low, probably 72ppi giving you say a 30 inch by 20 inch photo that if you resized by changing the ppi would make the photo smaller without actually changing the size via the dimension drop down in the 'edit-file size' menu.
My advise is to learn the crop tool and crop your photos if you want a 8x6 (4:3 ratio) this will certainly go a long way to the printer giving you what you want.
Or just accept your photo ratio of (most likely) 4:3 and print to this size, 6x4 or 9x6 or 12x8 etc and then no cropping is needed.
This advise is far from complete Ruth as people will come along with the ppi vs dpi scenario as well.
Just remember ppi is screen resolution and photo file size resolution and dpi is print resolution.
DPI dot per inch and PPI pixels per inch.
dpi is what a printer prints and is pretty simple, it gets confusing when people mix the two and some people give the wrong advise.
ppi on a 24 inch full HD monitor will be different to that of a 42 inch full HD TV.
They both have the same resolution 1920x1080 that's width x height
However, the TV at 42 inches has bigger pixels (not dots) to cover the larger screen and so its ppi will be lower.
