You're not clueless, just inexperienced! You'll get the hang of this!
What I would do is fix the dimensions by adding borders. It's perfectly fine to add wider borders to the top and bottom of the quilt than to the sides. So, for example, if it finished at 60" x 70" and you are okay with the 60" but need it 10" longer, just add a 5" border to the top and the bottom. If you want to add narrower borders to the sides so they match the top and bottom, go ahead and do so.
One of the great things about quilting is that it's your quilt; the only person you have to please is you. There are all kinds of things that happen when we're creating, and sometimes they take us down interesting new roads.
Have fun with it!