[@Blueflame][@KatherinWinter]

Here's my thought process. If his weakness to sunlight is a result of ultraviolet radiation, make it so he assumes the SHAPE of his final form at midnight every night, but is still heavily restricted on power; call it a maximum size of 10 feet tall. The reason being that even at night the moon is reflecting small amounts of UV radiation towards earth.

During astrological events which further reduce the amount of ultraviolet radiation present, allow him to reach a larger size and greater strength. Still less than the 30 foot mark, but larger than the 10. Maybe call it 15' tops.

A good example of a condition with even less UV radiation (at least, naturally occurring UV radiation. Most man-made light sources put out some UV light) would be a total lunar eclipse; a blood moon. During that particular event the earth is directly between the Sun and Moon. That means the only light reaching the moon (and thus being reflcted down to the surface of the earth) bis long-wavelength light on the red/infrared end of the spectrum which refracts through our atmosphere. Almost no ultraviolet light reaches the side of the planet facing the moon.