>>25158220-25 watts per squarefoot is a good rule of thumb.
Your two 20w tube lights should be pretty solid at getting a flat of something going.
The biggest CFLs I've been able to find locally are 45 watts. They kinda fit in those reflectors, lots of the bulb sticks out. Here is an old pic of some snap peas and yellow pear tomatoes I had going under a 45 watt. It was more than enough for them.
