What does "dimensionality of power distance" mean?
Hofstede has developed a cultural dimensions theory to compare countries, which uses factor analysis to assign values across 6 dimensions of culture. 
These dimensions are:


Power Distance Index
Individualism vs Collectivism
Masculinity vs Femininity
Uncertainty Avoidance Index
Long Term Orientation vs Short Term Orientation
Indulgence vs Restraint


That's all their seems to be when it comes to defining the term 'dimension': a measurable aspect of a situation. As for power distance, as you have already quoted as "the extent to which the less powerful members of institutions and organisations within a country expect and accept that power is distributed unequally", basically means how readily the people of the culture accept hierarchies and inequalities of power. A larger power distance would imply a wider gap between people at different levels of the hierarchy, and greater acceptance of this gap with lesser questioning or asking for justifications.