From the print works (the manga, etc. Mostly Wild and Horned Hermit, IIRC)
Hecatia is one of the highest levels of power in hell. Even Shikieiki remarks that she was one of its top authorities, and Hecatia's mentioned at various occasions that even if other denizons of hell might wish to cause disaster, they wouldn't dare do it while she was around. We don't know how she compares to Okina, but it is very clear that her power is supposed to be freakishly crazy. I guess most people, me included, have trouble wrapping our heads around that three bodies thing as any more than just a gimmick, but I think when ZUN said she was the most powerful, he figured that had to be the case if she was going to be a goddess of three friggin' worlds at once. Sure, Lunarians are powerful, but Hecatia's a goddess of the moon. And earth. And Otherworlds. So.... yea.
Ignoring Okina's relative power to Hecatia, it's important to note that ZUN intends for Okina to be thought of as insanely powerful too. Blatantly spelled out in her Wild and Horned Hermit chapter where it's revealed her sheer power alone was enough to make Komaru COME TO LIFE as a SIDE EFFECT, something that leaves even Reimu speechless in awe. I'm not sure Reimu's ever been portrayed as being completely utterly speechless at someone's power before. Even when dealing with Lunarians and Yukari, Reimu's always so rebellious and stubborn and "Whatever. I'll just exterminate you.". Heck, she was even that way with Okina throughout most of the chapter constantly telling Okina that if she caused any trouble, she was just going to have to beat her again, until that big reveal at the end, where for the first time in the series (I think?) Reimu seems to consider herself out of her league.
(that's not proof that Okina is necessarily more powerful than Yukari or anyone else by the way. Again, Reimu was just as pompous and prideful towards Okina as she was towards everyone else until she got a glimpse of just how far Okina's true power went, so there's always the chance that others simply haven't shown Reimu their true power either. It's just that, as this is the first time ZUN actually had such power be glimpsed, Okina is clearly meant to be seen as insanely powerful)
So.... yea. They're definitely meant to be seen as ridiculously powerful, even by the standards of the series.