1.5 DIN size is 1 and a half times the size of a standard head unit (single din)
most current model toyota's that dont have a double din deck usually have the 1.5 din unit with a blanking plate attached above it, as most of the 1.5 DIN toyota units have a 6 disc upgrade option which replaces the blanking plate
(not a cheap option though was somewhere around $600 from memory)