you COULD just cut the extra part off. But that means serious hemming and stitch catching to stop it from unraveling completely. And seeing the use a baby blanket will get, I would not recommend that.
Rather knit it again - or make it a big blanket eventually used for an older kid or adult or couch or chair... now that you have done so much work, you might as well put the rest of the work into it.
If you knit that into a square it will be about 5 feet each way. Nice couch plaid, if you ask me!