Assign value based on 100s of switch statement
If state.dimensionType is always an exact key match for a value in state.dimensions, and assuming that the default means the key doesn't exist, then you can do a lookup, for example:

if (!state.dimensions.hasOwnProperty(state.dimensionType)) {
  throw new Error('No dimension type specified');
}

state.dimensionQty = state.dimensions[state.dimensionType];