觀無量壽佛經義疏 3.124

3.124

「『先當觀於一丈六像在池水上。』」順此方機，故觀丈六。不言坐立，應是立像。二、後現大身。

"‘one should first contemplate an image sixteen feet tall upon the water of the pond.’" To accord with the faculties of this world, one contemplates the sixteen-foot form. It does not mention sitting or standing, so it should be a standing image. Second, afterwards manifesting the large body.