Ok let me see if I can give you some answers
1. Size - if you have the space and materials go bigger. Maybe 4 x 6 x 7. It will help with heat
Also if you haven't got one already get a cooltube fitting for your light as this will help eliminate some heat at the source.
2. Pots - if you can go with superroots airpots as in my experience these give the best results. Smart pots are the next best thing if you can't.
It's roughly 4litres to a gallon so 3 gallon would be 12 litre, but if you have the space go for 15 litre pots and super autos to get MONSTERS
3. Strains - so far the biggest yeilders I have tried with good power are think different by Dutch passion, chaze by flash, and Afghan express by positronics.
Another big plant that I have just ordered is auto extreme which is a collaboration by dinafem and Dutch passion, but I haven't tried these yet so I can't comment on power
good luck