I don't think you did anything wrong!!! Baby blankets can be pretty much any size out there and still be OK!!! Those babies grow up eventuallly and the blankets will get used no matter what the size is!
I have just started making all my baby blankets longer than what the pattern calls for...........simply because I want the blankets to get used for many, MANY years!!!
Keep your blanket the length it is, weave in the ends and ENJOY!!!