Personally, if you can fit a bigger tank, I would. I have 250 and wish I could have gone 300 with 2 adults and 2 kids.
That MCS guide doesn’t take into account how slow heat pumps are to recharge (compared to gas).
So the more usable you can get in one go the better for me.
This ‘mixing’ thread might be useful
And maybe my Mixergy performance thread.
Basically, emptying a tank and then reheating is better for performance.
as will lower target temp.
You can always top up when needed out of your normal day to day usage.