twin crochet afghan

I want to make a crochet twin size afghan using the afghan stitch.
How many chain stitches do I start with, and how many rows do I make?

I have several balloon cross stitch graphs I want to use. How large should they be for this size afghan?

Another question: I would like to use a bamboo afghan hook with an extender. I have never used an afghan hook with an extender before and was wondering if the work well?

Thank you for any help you can give.