I don't know what has happened to Oliver, he seems to have disappeared on us, however in the past I have built my FT outside by building a frame work of 4"x4" post 3' or so in the dirt and about 30" above ground. Then I use 1"x6"s to build the sides above ground. Then I dig out the interior into the dirt the desired dept going from shallow to deep to encourage the poo to the deep end to be pumped out. I then mold and smooth the dirt interior to suit my taste then line the dirt portion with used carpet foam padding. The I line the entire interior with 10 mil black plastic (available at Home Depot 100'x20' for about $200.) (You will have plenty left over for lining the the grow beds for raft systems.) I put 1"x6" cap rails on the top all the way around to make it presentable and to hide the plastic that is tacked to the top of the top board.

As for how big to go...I would go as big as you can afford...regardless of how many grow beds you have gallon wise as long as you have the bio-filter power to convert the ammonia and nitrites to nitrates, that's the major issue. With a big FT, you can always expand your grow beds as you please, but if you have a small FT, you will have to increase it's size in order to be able to handle more GBs. If you have more FT than recommended for the GBs and the plants aren't using all the nitrates, no problem...I have found nitrates to be relatively harmless to the fish (I have never had a problem with nitrates being too high myself) but you have to get rid of the ammonia and nitrites. They are the killers.

I hope this helps.