After a lot more testing....it's smart to set this as follows:
Samplerate / 10 and then select a power of 2 higher then the number you get.
So 1MSPS divided by 10 gives 100000, the nearest power of 2 is 131072...so you should set that number.
If you have 10MSPS, then it's 1000000....so the size should be 1048576
With the RX888 at 30MHz = 60MSPS = 6000000 => so 8388608 should be used.
Typical this should give a good number to start with.
Table Power of 2 found here: https://www.mymathtables.com/numbers/power-exponentiation/power-of-2.html
Setting this number too high just eats some memory, setting too low gives poor audio, stuttering, low performance or even crashes.