allow jobSize to be as low as 512 KB

allow jobSize to be as low as 512 KB

previous lower limit was 1 MB.

Note : by default, the lowest job size is 2 MB, achieved at level 1.
Even lower job sizes can be achieved by manipulating this value directly,
or manually modifying window sizes to lower amounts.

Updated unit test to ensure that this new limit works fine
(test would fail with previous 1 MB limit).