This is so easy and short that I didnt want to make a new page for it soo will put it here!!

 First... you cannot make a brush over 255x255 pixels.

 If you can put it on a canvas, you can make a brush out of it!! Open up a 100x 100 canvas, put whatever you want your brush to be on it, now use your majic wand and click somewhere inside the canvas.. just like the selection excersise anw go to "selections invert" so the ants are just around your object.... now choose your paint brush and right click for the "tool options box" now click on the paintbrush symbol and select "custom" and when that dialog box opens just hit the CREATE button.   Thats all there is to it!!!    you should now be able to see the list of custom brushes and your new one should be there!!!   Ollie