Depending on the program you're using to make it, you could select all of the white wool, give it a slight red tint, and then change the hue until it's blue. That would save you some time. If you're using miner shoes or something to make it you could also just use the randomization tool.
I can't do it myself because my CPU is being overloaded by world painter.
"wow, what did you make exactly for your custom world?"
According to world painter it takes one real day and night of pure walking to walk from the center (the spawn point) to the other side of the map. Meaning that it takes two days and two nights to traverse it all, from one side to the other. That should give you a good enough indication of the size.