Thanks for posting about this problem. Glad to hear so many students are using Google Charts.
I believe what you are seeing is that the bucketSize option is not actually ignored. If you change your example to set the option to each of 1, 2, 3, 4, and 5, you will see the chart changes, but it doesn't use the given bucketSize exactly unless it is close to a tick value. Rather, it effectively rounds to the nearest tick value, or something similar to that. I have to admit, this is a bug, since you should be able to specify any bucketSize value.
I don't see any workaround at this time. It is possible to specify options that determine the hAxis ticks, and those will be used, but only after the bucketing has been done. If the numBucketsRule allowed you to specify a function that you could provide to determine the number of buckets and thus the bucket size, that could also work.
But it will be easier, I expect, to just let the bucketSize option be used as specified. I'll look into that.