Does this mean the maximum size is roughly an X51?

"Although X is the last letter, there are flares more than 10 times the power of an X1, so X-class flares can go higher than 9. The most powerful flare on record was in 2003, during the last solar maximum. It was so powerful that it overloaded the sensors measuring it. They cut-out at X17, and the flare was later estimated to be about X45. A powerful X-class flare like that can create long lasting radiation storms, which can harm satellites and even give airline passengers, flying near the poles, small radiation doses. X flares also have the potential to create global transmission problems and world-wide blackouts." (1)

(1) https://svs.gsfc.nasa.gov/10109

How much time would elapse from the point of seeing the sunspot until the flare/CME actually occurs?

Hard to say. For a "super" event the sunspot group might be noticibly big right after it appears and flare during the emergence which would mean a ~day or two warning all the way up to a long established group slowly growing over an entire rotation around the sun and coming back to flare and/or erupt ~29 days later.

So, does that mean we'd have time to shut off our power grid and disconnect any long lines of metal?

AIUI, it's only a problem if sufficiently long wires are actually connected to vulnerable equipment.

Yes. But I think there'd be a lot of political/social problems with that. The solution for the grid at least is to spend the infrastructure money to put fast ground fault interrupters on all irreplacable (<months) big backbone transformers.