If you want to maintain the classic HD bagger/cruiser look then a tombstone shaped windshield is the common way to go. There are selection guides on websites like Revzilla and Bikebandit.
If you go that route you can just get a big one, that is a little taller than your eye level, and that will knock the air out of your way. The drawback is you have to look thru the plastic, and when its raining or covered with dirt and bugs it will be hard to see. It will also load up your engine noticeably and put a big drop on your mpgs.
The other approach is to get one that is about as high as your nose. You look over the top and it throws the air up over the top of your helmet. At highway speeds (70mph or more) these can be difficult to set up just right, and you may need fork deflectors or "lowers" to get the air to flow up and over your helmet so you dont get any head buffeting.
Head buffeting is not just air hitting your helmet. When you ride with no windshield you get a nice steady flow of air over you and your helmet. The faster you go the force increase by the square of your speed, but it is always steady air (unless you get behind a truck). When you put a windshield on your bike the flow is smooth off the edge of it, but some distance back it breaks up and becomes turbulent and chaotic. If that turbulent air hits your helmet its like your head is in a paint can shaker - your eyes wiggle so much your vision blurs and your brain is being vibrated in your skull. You will not be able to read road signs, and you will get a terrible headache in a manner of minutes at 75mph. You can NOT ride like that, you are literally getting a mild concussion from your head being shaken like a bowl of jello. You never get this effect riding with no windshield - it is worse than having nothing on your bike. If you slow down to 60mph or less it stops. Again, it can take quite a bit of adjusting the angle of the windshield and the position of the fork deflectors to get an acceptable ride at 80mph with this setup. Not a great ride, just ok.
The other way to go, if you really want to get on the interstate highways and haul, is to put a bat wing or a full fairing (like a Goldwing or Venture) on your bike. It will no longer look like a 1960s HD cruiser, but it will put you in a nice still pocket of air with no head buffeting, and you wont have to play with the exact height of the clear part, or mess around with fork deflectors.