I asked the same question about 6 months ago, and basically any way you slice it it will work pretty much.
I wound up hacking mine in a variety of ways. If it is too tall but the diameter is ok, amputate it half way up the stalk, and then cram the top half into rocks and it will attach itself in a few days. The bottom part will either die or will become a new toadstool.
If the diameter is too big, you can either cut pie-like wedges out of it, and each of these will grow, or you can cut a stripe around the circumference and make a donut, and cut these up into approximate squares, each of which will grow, etc. The idea here is to leave the original colony in a state where it can eventually grow back and retain its round appearance. It will look pretty mangled for awhile.
I put one of my quarter-sized cuttings into my 10 gallon nano and now I have a new mushroom coral that is about 5" in diameter and has an approximately 6" stalk.
It is best to do this with a sharp razor. I really worried a lot about doing this but it is hard to mess it up too much. Most if not all of the cuttings will live and will probably be looking good within 2 or 3 weeks.