Dogwood has big flowers and big leaves. Yes the leaves will reduce some with time in bonsai culture, but they will still be pretty big. Therefore, you need to grow them into bigger bonsai. A yard is probably the height you want to shoot for, and then the trees will look in scale. As with most deciduous bonsai, it is best to either start from a cutting, seedling, or airlayer and grow it slowly over time, trying never to make a big cut if you don't have to.
I think it's great you're growing dogwood for bonsai. We need more people using deciduous natives, and the Midwest had plenty of natives with good potential waiting to be utilized.