Right
若 supplies the sound — ruò = ruò, no shift.
若 also brings a faint flavour ('like, in the manner of') bleeding into
偌's 'such, so' reading:
偌大 'so big' has a 'man-like-this-size' feel. Mostly literary; in modern Mandarin you'll see it almost only in
偌大 and
偌多.