I'm pretty sure they can grow to be bigger, like I said I underestimated the size for Anguirus as being smaller since nearly all Singular Point Kaiju are tiny but I'm pretty sure the Skullcrawlers as awhole can get much bigger (as shown with Ramarak and the GvK Skull Crawler)
So size isn't much of an issue for the Skull Crawlers, the Juveniles themselves can also grow pretty big considering they were near Kong sized