depends how big the plants are and the heat of your room as to how much they will need. also depends how long you want to go without changing reservoir.
what he said ftw! 10-15 gal w/ auxiliary 5 gal bucket at cieling feeding drip lines. with a drain set at a certain height in the 5gal that waterfalls back into the main rez(i usually put the top of the drain about 8" from the bottom of the bucket giving me around 1.5 gallons in top rez at all times, which maintains constant line pressure and prevents things from going out of adjustment due to fluctuation in water pressure) this is the only way that I have setup my blumats and felt comfortable leaving for any given period of time. my main rez is on a float valve and gets fed fresh filtered tap as the level falls, usually after a few days I turn the source water off and use up whats left to prevent things from getting too diluted.What most do is use the 5g bucket raised up, but have a larger res on the floor with a small pump that tops off the bucket.
You have an overflow run back down to the big res like one big cycle.
This way you never touch the top bucket, just fill up the bottom res when you think you need it. You will also keep constant pressure going to all the blumats,and have less chance of a runoff.
If you do some blumat searches you'll find some exact examples of this.