whatever number of plants you do decide to use wouldnt use more than 1 plant per sq ft of screen and in a scrog you will have to veg a little while in the bubbler this amount can vary alot depending on how many plants you use if using a 2 by 2 screen you may want to use 4 plants for minimum veg time.
as per the size would try to get a tub large enough that when filled to a normal level just below the netcups it would hold atleast 3 gallons per plant used so if you used 4 plants a 12 gallon, but you can go larger or smaller in size just a smaller one would require more maintence.
you should read up on scrog and bubblers and make sure its something you want to do before building one, will post a link to og faq at the bottom of the page. the scrog that ive seen done using a dwc tub all had a screen attached to the lid so all the plants and screen could be lifted off and nutrients could be changed out and having a spare tub to alternate out makes it a little easier.
http://www.drugs-forum.com/growfaq/G...c%20Topics.htm