Full sheets of watercolour paper often come in Imperial sizes rather than European A (ISO) sizes.
A sheet of Imperial (also called Full Imperial) is 30×22 inches (76x56cm). This is a bit smaller than A1 which is 84.1×59.4cm.
The other A sizes are based on half of the larger size above it:
A2 is 42×59.4cm
A3 is 42×29.7cm (which is the size of our catalogue open)
A4 21×29.7cm (which is the size of our catalogue closed)