You will see the option of imperial and metric sizing.
The overall size is very similar but for framing purposes I have given you two options.
IKEA usually stock 50 x 40 CM frames. Whereas 20 x 16 INCH frames can be found widely in most High st stores.
Size includes mount, so just look for a frame the same size as you order and the print with mount will fit straight in.
Ref: YARMIL AP