How Do I measure a belt for pulley swaps

How do I properly measure for belt size for a pulley swap?
How much play should I have with clutch disengaged?
I plan to step the trans pulley down to a 2" or so so I'll need a new belt, but the original swap which this pic is about a month after the swap being done, I had terrible slippage under load, even climbing hills, I really want this to work well

Thanks!

Belt should be tight enough that the clutch pulley should still have tension on it.  Should not flop.  As for measuring, you can take an old belt (larger) that is no good, cut it, run it through your pulleys and attach the ends where it is tight.  Take that to the store you buy belts. They should have a belt measurer there.  Another method is to use a piece of rope to do the same.

I've used the method Taryl uses in this video.  Fast forward to 41:31.

I thought about using large zip ties but if I measure clutch engaged will it fully release when clutch is pressed? That was also a problem I had last time

I would measure when the clutch is released.  Also, I would think a zip tie would be too rigid for an accurate measure.

Fair enough, I'll try that Thanks

Do you mean released as in tension on belt or tension off belt

Only 1 problem I don't know what the original belt size was
There wasn't a guide on the mower anywhere