If you're thinking of converting a bar recipe, just consider the area of the pan you're using when deciding whether you need to make a 1/3 batch, 1/2 batch, etc. So this recipe for apple crumble bars uses a 9"x13" pan, which has 117 square inches. If I only wanted to make an 8"x8" pan, which has 64 square inches, then I would need to halve the recipe. (Noting that, because the pan has just slightly more than half the area, a half recipe of bars will be spread just a tad more thinly.) I have halved this recipe many times for an 8"x8" pan and it just means that the crust, apple layer, and streusel are slightly thinner/shorter layers.