code: func (f *Flex) ResizeItem(p Primitive, fixedSize, proportion int) *Flex {
  	for _, item := range f.items {
  		if item.Item == p {
  			item.FixedSize = fixedSize
  			item.Proportion = proportion
  		}
  	}
  	return f
  }
docs: // ResizeItem sets a new size for the item(s) with the given primitive. If there
  // are multiple Flex items with the same primitive, they will all receive the
  // same size. For details regarding the size parameters, see AddItem().