This new model is expected to be released later this year. From what I've read it'll be about the size of the '97-'03 A4. As more details are released, please post them under this topic.
Per an article in the October edition of Road & Track, the 2015 A3 1.8 sedan is better than the soon-to-be-introduced Mercedes GLA, in terms of handling, roominess and blind spots. The A3 sedan is scheduled to be introduced in Spring 2014 in the U.S.
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They mention the S3 variant coming but are a tad bit vague about guaranteeing it will come to the US at the same time as the A3.