In the example we are discussing (EURGBP - 9th March), I guess I don't understand how you are calculating lot sizes.

These figures are roughly correct
So we sell 0.01 at 8975 20.30hrs 6th March
Sell 0.01 @8970 01:00hrs 9th March
Sell 0.01 @8969 04:15hrs 9th March

.....then we get the big market rally up to @9186 but the PSAR is triggered again @9160

Our latest entry before the rally was 8969 so 191 pips (9160 minus 8969). 191pips divided by 30 is 6 discarding fractionals.

So how do you arrive at the next trade being 0.64? Is this because you follow the 0.01, 0.02, 0.04, 0.08, 0.16, 0.32, 0.64 sequence? So if the last trade had been 0.04, the size traded would have ben 2.56?

You could end up trading some big lot sizes here.