Rule of thumb here for two drives or more in the Drobo is that the largest drive is used for redundancy only, and you get the sum of the others as usable capacity.
Your current setup of 1.5T/1T/750G/250G should therefore give you 1T + 750G + 250G = 2T of space (the 1.5T goes to redundancy).
Swapping the 250G would give you 2T/1.5T/1T/750G, giving you 1.5T + 1T + 750G = 3.25T (the 2T goes to redundancy).
Swapping the 1TB would give you 2T/1.5T/750G/250G, giving you 1.5T + 750G + 250G = 2.5T (the 2T again goes to redundancy).
All of these calculations are in “disk drive manufacturer” decimal-based GB/TB, your OS and Dashboard may report in “operating system” binary-based GB/TB, which will make the numbers there look smaller.
If you are looking for capacity, then switching the 250G is the clear winner. If you just want the 1TB drive out to have a same-size drive to back up your other 1TB drive then you will some capacity gain in the Drobo, just not as much.
(Of course you could always swap the 250G, and put the backup of the 1TB drive on the Drobo instead. You will gain 1.25T on the Drobo when you put the 2T drive in instead of the 250G, which is enough extra space to hold the backup of the 1T drive and give you an extra 250G to play with too…)
They have an on-line calculator at http://drobo.com/resources/drobolator.php that lets you play with various combinations and see what you get.