Magento Redis setup on AWS

For #2, Magento creates a session for each visitor; to get an estimate of how much space you need, just multiply the size of one session storage by the highest number of visitors (and multiply by.. let's say 4-5 or maybe even 10, to be safe :D). You can also setup alters in AWS, for when you reach like 80% memory usage. Also, you can check if garbage collection is working and play with the expires.

For #3, see this response on SO https://stackoverflow.com/a/8654367/64944

php-redis is a c php extension, it will perform better than native php. If php-redis is available Magento will use that for its communications with redis, if not it will fall back to a native php implementation. You don't need to use php-redis but if its installed you might get slightly better performance.