There is no rule that says “you must sit in the front row of every class.” In fact, I almost never sit in the front row in class.
If you have trouble seeing, it would make sense to sit towards the front of the class. Where you sit in class (most of the time) will not make a difference in your grade. What matters is your engagement. Often times, people sit in the front row to pretend to be engaged but in reality they are distracted by their computer, phone, etc.
In a larger lecture
Sit near a light source. When the lights are dimmed, it is easy to begin to fall asleep. So instead of sitting in the front where it is the most dim, sit underneath the light to help you stay awake. Even though you are not in the front of the room, feel free to ask questions and participate like normal.
In a medium sized lecture
Medium sized lectures follow similar rules as a larger lecture. Sit in an area that you can see, be engaged in the lecture, and participate when required.
In a small classroom
In a small classroom, choose a seat that will not draw much attention (third row area). This way you are close enough to participate without feeling overwhelmed but you also will not be constantly called on.
Lastly: no matter what size the lecture is, if you have to leave at any point during the lecture, sit near an exit. Please be mindful that it is distracting for your professor and peers when you leave.