Welcome to the Boards, and congratulations on your beautiful pup.
There is no "right" size for a border collie. Until recently they weren't bred for appearance at all, just for working ability. So if a small dog could work its sheep proficiently, well, then it was the perfect size. Ditto for a larger dog.
At 13 months Olive isn't likely to grow much more, if at all. Border collies tend to mature at a younger age than many other breeds, so as a female she's likely come close to her maximum growth. She may fill out a little as she matures, but unlikely she'll get any taller than she in now.
I'd worry more at this point if you try to get her to eat more she'll just put on too much weight. Make sure you can feel her ribs easily to maintain a healthy weight. Feeding her more won't make her grow more in over all size, just in girth and that's not a good thing.
I'd love to have one that size. If you decide she's too small for your liking, I'll take her off your hands. (JK . . . sorta.)
p.s. I used to live in your area, in New Tripoli, nearer New Smithville. Worked for the Northern Berks & East Penn Valley Merchandisers so am very familiar with Fleetwood. Is the hardware(?) store with all the stuffed game animals still there? It used to make be cry to see all the wolves, killed for nothing more than to be trophies.