My first crochet project as a teenager was granny squares and they're great to work on as a beginner. It's also a good way to use up small pieces of yarn by either making small granny squares and joining them or continuously for a blanket or lapghan.
Pattern and instructions for a rectangular granny lapghan here (I would start with 22 or 33 chain though for a lapghan) which should give you an approximate size of 39 x 48