For GPU memory monitoring (and lots of other GPU monitoring) use the GPU-Z app from https://www.techpowerup.com/gpuz/ (Beware there are counterfeits on the web that are Trojans. Only download from the techpowerup web site.)

As for the actual bucket value it's not used for Progressive render mode. I highly recommend trying some combinations for your self. I every scenario I've tried progressive always comes up as 5-10% faster. And if I user buckets the smaller they are the slower it goes. It's fastest if it's one big bucket.

I suspect the only case where using buckets would be better is if you have two GPUs. Then they can both be rendering at the same time.