well, which guitar? to be truly picky with a proper answer, it would properly depend on the scale of the axe in question (assuming an RG probably, but it does matter if we're being technical )
but in any case, probably at least 10-52. nowadays drop c almost seems like a high tuning compared to a lot of the stuff "kids these days" are into, but it definitely is a very noticeable shift when using the same strings, especially with the bottom.
some people roll with as thin as a 9 even for drop c and don't mind it, but 42 is kind of iffy for that low end, in my opinion.
10, 11 or 12 thru 52 to 54ish range is i think what most people like for drop c on a fender or ibby scale.
if you have something a little easier to string and tune up than a double locker, you might grab a couple diff sizes of cheap sets (say, 10-52 and 11-54) and just give a day or two with each set...(on a strat or AZ, perhaps?), and see how you like the tension on each (of course RG scale isn't the same as strat anyway, but nonetheless, should give you a rough idea for how it feels).
if that sounds like more effort and money than it's worth (and/or you just want to dive in on the ibanez you got your eye on, and set it up properly from the start) and you just want to pick one set and see how it goes, i'd probably go with 11-52 or 54. that way, if you feel like it's too heavy you can go for something like 10-52 next time to see how you like that better, or go thicker if you feel the other way.
of course, as you may have already considered as an option...you could also just re-set up the ibanez with the existing 9-42 in drop c and see how you feel about that...a day or so would likely give you a good idea of how much thicker you want to go than that. in other words, if you felt like 9 wasn't too far off from what you're expecting, then you'd know to maybe only go for a 10 there, etc.
That's true... I could just try it out with the 9's I just had a feeling they'd be too floppy. I am one of the blasphemous Ibanez owners... I don't own an Ibanez with an Ibanez neck 🤨... I was going to use my JS-AZ fusion guitar. The neck is a warmoth standard thin C shape... Definitely thicker than most Ibanez necks. It's probably close to a road star.