My neon green toadstool is about 10-12" in diameter, with about a 4" stalk.
At that size its growth has seemed to slow a bit.
Over the past 7 years, for space reasons, I would trim it when it got to 8-10" ... either by cutting the whole head off at the top of the stalk ... or by removing the bulk of the head in a doughnut shape using the top of the stalk as a guide (leaving head in polyps only at the top of the stalk.
They're very hardy.
When you frag/trim it, it will seem dormant and be closed up for 2-4 weeks. Then, all of a sudden, new polyps will start to appear and the growth cycle will begin again.