Hey this is Pays164, and this is how to balance your letters in your graffiti piece. Make sure you just kind of step back and look at the wall, get your proportion down. If you're working with a sketch feel free to draw a line in the center of that and have that your baseline you're going to build up off of. Once you've stepped back and created that center path that you want your piece to follow.
Just come through, do the same thing with the bottom, and the same thing with the top. You can notice I'm just barely hitting the wall. You don't want to put any really solid lines that maybe some pieces of your graffiti mural won't cross over and you'll have to come back later for clean up. Now you can also avoid having to put a lot of these lines if you're working with a brick wall. The bricks can serve as a guideline. Simply starting from a center brick, you can count your way out on both ends and get everything proportioned real quickly, other than that a little light sketching and you're ready to rock.