Hey, you're with [Pays164], and here's some tips on creating a large mural. First and foremost, your canvas; make sure the wall you're going to pick is at least going to be somewhat smooth. Try not to deal with too many cuts and crevices in a wall. If you're going to take the time to do a large mural, you want to be able to do the details you want to get it nice and crisp.
Second, the priming of your wall; pick a color that's going to be a good base for the design you're going to lay down. If you feel you want to do something dark in the color scheme of your graffiti piece, character or background, black is typically a good color to start with. Satin, definitely soaks up the paint a lot better, so go ahead and start off with a good satin black. Maybe even other dark colors like a stone grey, dark blue.
Other tips on creating large graffiti murals, is your spacing of letters. Get a good focal point in the center of the wall to line yourself up. Center off and create spaces that you know are going to fill up the space for your letters. For instance, my name Pays, four letters; I'm just simply going to create a very light line across the wall, in which I will come back and pick my center point. To drop it, center it again, and center it again. Just that alone is going to get me my bases of how I'm going to take my four letters in Pays, and break them down to use an even amount of space on my big mural.