I can't really speak for gains in size or strength, but I know for competitive MMA fighters, roids can provide a real edge. The time taken for recovery whilst 'on' is hugely reduced, meaning you can train longer, harder and more frequently. I think the most common thing that fighters do is small amounts of certain substances, which do not cause significant bloat or water retention, as weight classes are so important. It is also important to take a sufficiently small dosage as to not elevate BP levels, as high BP will kill a guys cardio. Also, remember to keep it to substanceds with a short half life, if there's a possibility you will be tested at any point!