The full size (full size) and double (double) are synonymous terms generally used to refer to exactly the same bed. However, exactly what is the meaning of full size daybed and twin may still vary depending on where in the world you are talking about it. From continent to continent and country to country, full size beds or double beds vary in size a little.
In North America, the standard full size daybed measuring 54×75 inches (137, 16×190, 5 cm). However there are variations of this standard. For example, the full-size sofa bed measures 54 inches (137.16 cm) by only 72 inches (182.88 cm). Additionally, there is a full size bed XL measuring 54×80 inches (137, 16×203, 2 cm).
Depending on location in Europe, full size daybed varies. In most of the European continent full size beds measuring 55×79 inches (139, 7×200, 66 cm). However in the UK and Ireland standard full size bed measuring 55×75 inches (139,7×190,5 cm) while a twin size bed is available and measures 48×79 inches (121,92×200.66 cm). Norway has not formally referred to as beds full size but offers what is called a large simple measuring 47 x 79 inches.
Asia offers a double bed measuring 55×77 inches (139, 7×195, 58 cm). Only Thailand seems to break the mold with its full size bed with measures 48 x 78 inches.