Gauge

You need to work a sample swatch using the hook and yarn for your project. Do a swatch that measures 6-8 inches square. Then measure the number of stitches in the middle over 4 inches and the number of rows in the middle over 4 inches to determine how many stitches/rows per inch. With this information, you can calculate how many chains you need for your foundation and how many rows you should work.

If you are doing the basic afghan stitch, it will curl up, so you should also plan to do an all around border of 2-4 inches deep so you need to factor the border depth into your overall size. So if you decide you will have a 2 inch border all around your afghan, this will add 4 inches to the length (2" for the bottom and 2" for the top) and 4 inches to the width (2 inches on each side.)

I'm not familiar with an extender so I don't have any thoughts for you. When I have worked a large afghan in afghan stitch in one piece, I used an afghan hook with a flexible cable end.

As to your balloon graphs, again, with your gauge you can determine how many and where you want to place your balloons.