Due to a decimal point error on the A5 paper size. A5 paper is not detected as A5.
to resolve this we create a new paper size with the correct dimensions.
steps will be similar for other canon printers.
Step 1: in "Devices and Printers"

Step 1 screenshot.
Step 2: "Canon LBP6230/6240 Printing Preferences"

Step 2 screenshot.
Step 3: left click on "Page Setup (tab item)" in "Canon LBP6230/6240 Printing Preferences"

Step 3 screenshot.
Step 4: left click on "Custom Paper Size... (button)" in "Canon LBP6230/6240 Printing Preferences"

Step 4 screenshot.
Step 5: left click on "A5 (text)" in "Custom Paper Size Settings"

Step 5 screenshot.
Step 6: "Width: (edit)" in "Custom Paper Size Settings"

Step 6 screenshot.
Step 7: change width to 148mm;Rename paper 'A5 custom'
Step 8: left click on "Register (button)" in "Custom Paper Size Settings"
Step 9: left click in "Custom Paper Size Settings"
Step 10: left click on "Apply (button)" in "Canon LBP6230/6240 Printing Preferences"
Step 11: left click on "Canon LBP6230/6240 Printing Preferences (dialog)" in "Canon LBP6230/6240 Printing Preferences"