What do you mean by big renders? Do you mean for printing purposes? If this is the case then I would say no. I just spent the last 30 minutes playing with flames in Bryce and the only thing I can really say is that the bigger you make the finished image the more it looks like blurry crap. Even changing the finished size from 3000x2250 at 72dpi to 3000x2250 at 300dpi didn't make any difference. It still looks like blurry crap. Bryce is great if you want to change the flame in some way, incorporate it into a 3D scene or turn it into a material. For trying to make bigger renders of basic flames it isn't really an option though. Maybe I missed something when I was testing it and someone else knows how to do this. I don't use Bryce in this way so i'm not 100% sure. Or try asking in the Bryce Forum...