A bend through the handle can be shorter than 72".
A 45# can be narrower but if it is rigid handle, I'd stick with the longer dimension. After you have made a few bows, you can start tweaking the design but if you want a safe, overbuilt, durable red oak bow that doesn't follow the string by more than a couple inches, go with 72".
Don't back a bow just because. If the back is good, a self bow will perform better. If the back is not good, then you should look for another piece of wood, IMO. Unless of course you just want to try it, then I highly recommend you go for it. This is supposed to be fun. Experimentation is fun.
When I cook a new recipe, I follow it exact the first couple times. After I gain experience, I play around some. But you have to build the foundation before you can start framing. Learn how to remove wood. Learn how different designs are going to work on woods. Learn to tiller. After you get those down, start playing around. There is no way you are just gonna build one bow anyway.
My very first bow was a pretty aggressive hickory that folded on me because I ignored or didn't recognize a hinge. The next 3 were Torges style and they are all still shooting. They aren't great bows but I sure learned with them. Then I made 20-30 successful bows that got better with each one and I don't think I broke one. Then I started pushing the envelope and broke about every one. 7 in a row if I remember right.
It's all fun. It's all good.