GeneferCUDA automatically picks the fastest blocksize setting when it starts up. The only time you really would change this is if it's picking the wrong setting, or if you want to manually de-tune it to reduce screen lag.
But yes, you can change it on the fly. You need to first change it on the website, then do a BOINC update to get the new setting down to your computer, then finally pause and resume the GeneferCUDA task to get it to take effect.
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