you didnt mention what power u will be feeding those subwoofers...
anyway, go for the biggest battery that can fit in your car. if u want a good sound, than get the biggest that fits the car's battery tray.
i have an 85amp exide in my charade 84 model...this is the biggest i can fit in the original space. and i am running about 900rms in my car.
do not get 50 or 60amp battery. rs500-1000 ziada kharch kerlo and get a bigger battery...