Felting

I know when I felt a knitted item that I loose approximately 1/3 of the size.  It is hard to make something so much bigger than I know it needs to end up.  I just felted a bag that I hope will fit a Kindle DX.  I'm not sure it will.  I'll have to do a "fitting".  And I really doubt there is room inside for the Kindle to stay in its case as was the request.  So.  I cast on again and make sure I add even more stitches and rows.  Meanwhile, I have a bag here that is just barely 8 x 10 inches.  Bright Red.