User blog:Triple-One/Help?

I just got the PaintTool SAI program, and I'm having trouble with it. I also have FireAlpaca, and I made a picture of a character. When I tried uploading it, it said it was illegal. If I remember correctly, at the end of the pic it says ".mdp". This is the trouble I have with FireAlpaca. The problem I have with PaintTool SAI, is that I don't know how to change the size of the brushes. Like in Paint, you can change the size of the brush, but in SAI, I don't know how to do it. I think I got the new version, I don't know. But, for the people who use PaintTool SAI, and FireAlpaca, can you help me with these problems?