Time constraints do challenge many of us to dig deep down and be extra-creative, but I believe that creativity also depends on the composer himself (or herself). For instance, I could still write a piece of music that's 5 mins long, or one that's 15-20 mins long, but if I'm just not a good composer, then it's a piece of crap regardless. The "fair playing field" is, in fact, that variation in time limit. Maybe some people are better at writing shorter pieces, and some people are better at writing longer ones; their level of compositional acumen will (or should) show through any constraints thrown at them. If 5 mins is enough for you to write a knock-out piece, then I'm sure the judges would love to hear it, and judge you fairly based on the content! Honestly, I'd rather hear a shorter piece that's halfway decent than a really long, atrocious piece...