In 3.x, you can do this by overriding the "_calculateClickTolerance" function of the map's popupManager property. For example:
map.popupManager._calculateClickTolerance = function(graphicsLayers) {
return 10;
};
That is a very simplistic example, but you could implement whatever logic you want inside that function as long as you return an integer (preferably non-negative).
The "graphicsLayers" parameter is an array of GraphicsLayer instances (this includes classes that inherit GraphicsLayer, like FeatureLayer). The returned value is the tolerance in pixels.
The default implementation basically does this: for each SimpleRenderer, ClassBreaksRenderer, or UniqueValuesRenderer found in the input GraphicsLayer array, it looks at each symbol in the renderer and finds the one with the greatest offset value (this includes "xoffset" and "yoffset" values). If the greatest offset it finds is greater than 6, it returns that. Otherwise, it returns 6 by default.
