How does the shape of a volcano relate to the average viscosity of the eruptive products of that volcano?
Higher viscosities will lead to less flow, and therefore higher peaks on smaller bases. The general conical shape will not change.
Viscosity is the resistance of magma to flow when subjected to a force. In general, the viscosity of magma increases as the silica content increases from basalt to rhyolite. http://www.gso.uri.edu/lava/MagmaProperties/properties.html