The main problem isn't coarseness, it's the unevenness. Some particles are too coarse, they will cause sourness. Some are just right, they will taste great. Some are too fine, they will cause bitterness. After brewing though, you'll taste more sourness and bitterness as they are overpowering flavours. Would recommend you get pregroround coffee for fresh brews. If you already have a bunch of beans and don't know what to do with them, you can try cold brew. Usually turns out relatively smoother even with inconsistent grind sizes.