1. To create a new Brush Dynamic, click one of the Paint tools: Pencil, Paintbrush, Eraser or Airbrush. Click the Dynamics icon.
   
   
2. The Dynamics list will appear and on the lower right, there is another, smaller Dynamics icon. Click that.
   
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3. At the bottom of the Paint Dynamics dialog are 4 icons. Click the Create a new dynamics icon.
   
   
4. The Paint Dynamics Matrix with various boxes that can be checked appears. This Dynamic will be a Random Size dynamic. Name the Dynamic. Checking the box at the intersection of Size and Random will create a Random Size dynamic.
   
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5. Close the dialogs. Your new dynamic should be in the list.

Note: Dynamics that come packaged with Gimp can not be changed. You will find the editing window grayed out for those Dynamics.

Here is an example of using Random Size dynamics.


The Gimp Manual explains the different terms in the paint dynamics matrix.
Gimp Manual - Dynamics

Download the PDF of this tutorial.

Oregonian, thank you very much for this tutorial you did. I was doing a little creation when I needed to use just the one you kindly thought and wrote. I followed your explanations and I got it, without any problem. Thanks!