I shot a big project whith the detail level on thick and I just heard recently that if something is shot with that setting, the image will look like crap when blown up on a screen.
Is this true? And if so, is there anything I can do in post to correct the problem?
It's definitely better to shoot in Thin than Thick if blowing up to film...but for all intents and purposes, I doubt that if you were completely dissatisfied with your footage shot in Thick, that you'd be considerably happier having shot in Thin...there is a resolution increase, but it's not like HD origination or anything.
In post, you could consider uprezzing your footage using Disjecta's frame-by-frame method using Photozoom....that definitely enhances the picture to a degree.
Thick is generally used for television production, as it has the most resolution NTSC is capable of displaying properly on most televisions. Thin gives you more detail, but adds a flicker to your footage when viewed on a TV. It's best for computer monitors or, in your case, a blow up to the big screen.