Try handling NodeCreated and setting Width and Height of node.Bounds to Math.Max(current value, min value), and also set the MinWidth and MinHeight properties of node.Constraints.

I hope that helps,
Stoyan

E.g. add this to the end of form_Clicked if you will allow creating nodes only through click events:

node.Constraints.MinWidth = 50;
node.Constraints.MinHeight = 35;

Note that this sets the node minimum size, and not the size of the child controls inside. Since the <rectangle> in your user control has very large margins, nothing is seen when the node has the minimum size, so perhaps you should add the margin values to the 50 x 35 size.

I hope that helps,
Stoyan