Inconveniently, the rules don't specify a size limit for organisms with passive respiration.
A quick check suggests sea spiders
use passive body surface-based respiration, so for now, we can use the maximum sea spider size of 70 cm as maximum provisional size limit in the best possible conditions of high surface area and cold, high-oxygen conditions, and a maximum of 50 cm in less-ideal conditions. In this case, no editing is needed, since it's 40 cm wide and uses the inside of its mouth for extra respiration area.
With that checked over, this looks good for approval.