Agave decipiens is an extra large species (several feet wide and tall) that I would not recommend long term in a window planter.
Just a quick ID note fyi, since some names may be confusing. At one point schidigera and multifilfera were lumped with filifera, but the latter is different from the other two (among other ways) in that it offsets regularly. However you find them labeled, this is an important practical distinction.
You can expect agaves to grow on the large side under the conditions you describe (afternoon shade, lots of water). So choose plants that are smaller to start with. Agave attenuata is pretty common around here in larger planters (it's the go-to agave in these situations because it is spineless) and it seems to handle constrained space pretty well, growing as a sort of dwarf version of the plants in the ground.
You might want to consider mixing in some aloes between the agaves. They would be compatible culturally and lend some seasonal color & hummingbird interest when they flower.
