How to print out an 6"x8" pic.

Frankly, no, but you might get somewhere by copying and pasting the picture file into a Word document, resizing it to the dimensions you want, then printing the page.

I know you have it sorted now but I just looked at the Windows 10 Print Picture tool and did see a 20x25cm option.

I opened File Explorer and went and found one of my pictures (it happens to be landscape rather than your portrait orientation). I also only have a black and white laser printer so please ignore the greyscale in the shot below.

Next I right-clicked on the picture and selected Print. The "How do you want to print your picture" window appeared. In it I could select my printer, the paper size the dpi resolution and the paper type. Down on the right side was a column of "frame sizes", one of which is 20x25cm.

Do you see that on your machine? Also make sure "Fit picture to frame" is selected.

There is an option to change Windows 10 from Metric to Imperial which will switch to Imperial photo sizes but it is well buried!

If you are brave enough to dive into the darker areas of Win10, it is Control Panel - Time and Language - Date and Time Regional Formatting - Region - Change Date, Time or Number Format - Additional Settings - Measurement System : Metric/US

Phew!