double itemExtent

The main-axis extent of each item in the list.

If itemExtent is null, the items are required to match the main-axis extent of the list itself.

Source 

double get itemExtent => _itemExtent;
void itemExtent=(double newValue)

Source 

set itemExtent (double newValue) {
  if (_itemExtent == newValue)
    return;
  _itemExtent = newValue;
  markNeedsLayout();
}