@sumaia – validateattributes simply ensures that the specified input variable is of the expected type. You can make these checks yourself, without really needing to use validateattributes. Or, if you are confident that the variables are of the expected type, you can skip these checks altogether.

Hi Foster,
This is another good situation for subclassing.
Here is a pretty minimal class for setting the vertex size. Note it does not handle the situation of adding a new vertex or changing vertex color after creation. For now you can just call the setVertexSize method after doing one of those operations. That’s slightly more sophisticated and may be worth a complete blog entry.

classdef impoly_vertex < impoly
    
    properties (GetAccess = public, SetAccess = private)
        vertexCrossSize;
        vertexDotSize;
    end %properties
    
    methods
        function obj =  impoly_vertex(crossSize,dotSize,varargin)
            if nargin < 2
                error('impoly_vertex:InsufficientArguments','Both the cross and dot size must be passed as constructor arguments');
            end %if
            obj = obj@impoly(varargin{:});
            obj.parseSizeInputs(crossSize,dotSize);
            obj.setVertexSize;
        end %impoly_vertex
        
        function setVertexSize(obj)
            crossChildren = findobj(obj.h_group,'Type','line','Marker','+');
            dotChildren = findobj(obj.h_group,'Type','line','Marker','O');
            set(crossChildren,'MarkerSize',obj.vertexCrossSize);
            set(dotChildren,'MarkerSize',obj.vertexDotSize);
        end
        
        function setCrossSize(obj,cs)
            obj.parseSizeInputs(cs,obj.vertexDotSize);
            obj.setVertexSize;
        end %setCrossSize
        
        function setDotSize(obj,ds)
            obj.parseSizeInputs(obj.vertexCrossSize,ds);
            obj.setVertexSize;
        end %setCrossSize
    end
    
    methods (Access = protected)
        function parseSizeInputs(obj,cs,ds)
            validateattributes(cs,{'numeric'},{'scalar','nonnegative','real'});
            validateattributes(ds,{'numeric'},{'scalar','nonnegative','real'});
            obj.vertexCrossSize = cs;
            obj.vertexDotSize = ds;
        end %parseSizeInputs
    end %private Methods    
    
end %impoly_vertex

Here we let it draw the polygon then find the vertices and change the marker size of the lines that make them up. Usage as follows:

figure, imshow('gantrycrane.png');
%about half the regular vertex size
h = impoly_vertex(4.5,2.25,gca, [188,30; 189,142; 93,141; 13,41; 14,29]);
setColor(h,'yellow');
h.setVertexSize; %need to reset manually for this version
addNewPositionCallback(h,@(p) title(mat2str(p,3)));
fcn = makeConstrainToRectFcn('impoly',get(gca,'XLim'),...
get(gca,'YLim'));
setPositionConstraintFcn(h,fcn);

A couple more points:

I really try to avoid editing any of the supplied code as it becomes a maintenance headache when you change versions.

Also a useful detective tool is the metaclass class

So mc = ?impoly gives you all sorts of useful info to help design you subclass.

Hope this Helps
Matt W