The best you can do with chaga if all you have are chunks is to just break the chunks into pieces that are small enough to fit into your pot. If you can get the pieces to be on average the size of one inch cubes then you will need to brew your chaga for about 12 hours to extract most of the tea available. Throw it in your stock pot over night.
I did a test on the difference in the amount of tea you get from chaga made from chunks versus chaga made from ground up tea. I was really expecting that the chunks would not give nearly as mush tea. I was actually quite surprised to see that in fact, if you brew the chunks for about 12 hours they make nearly as much tea as 2 hours of brewing the loose ground chaga.
So don't feel bad about the chunks. Chunks are good. You just need to brew it in batches. Chaga keeps in the fridge very well too so making a big batch isn't going to be a waste.