Fleece IME isn't likely to shrink but it will behave differently once the sizing or whatever it is in these synthetic fibers is washed out. It might actually be a little bigger. This is based on the assumption that your fleece is all synthetic. I've used fleece for PJs and sweatshirts and things and never noticed shrinkage. I've noticed that it's softer and more relaxed after washing.
Is your afghan washable and dryable? If so you'll want to wash and dry it before sewing them together. If not then prep it however it will be cleaned before you sew the two together, I think.