5 gallon works just fine, but if you want monsters go with bigger or in the ground. I believe 5gallon pots can grow a 4-5 footer easily, or bigger. FYI go to your local home depot, lowes, or menards(if you live in the northern states) and you can buy 5 gallon buckets for a little over $2 a piece, its a great deal compared to the normal $15+ for a 5gallon pot.
i said 1 gal per month of life indoors. get that pick ax going. best of luck
25 gallons is 3.3cf of soil. What size holes are you digging for how many plants? I ask because if your ground is already hard summer heat will turn it into brick material and small holes won't work as you'll have drainage problems. 2'x2'x2' holes are 8cf. If you run 12 plants that's 96cf or 718 gallons, a lot of soil amendments to move anywhere by hand.
For drainage, it depends on how deep the clay content goes. Many growers with clay based native soil prefer long, wide holes rather than deep, allowing summer heat to disperse through evaporation some of the contained water to avoid root rot as roots spread out rather than go down. Growers are now using 2-300 gallon Smart Pots which are only ~24" high for the same reason.
If you would rather not pay for SmartPots, or any other air root-pruning design pot, than you would also rather not have the best growing most productive plants you can have.
You can put your grow medium together with components such as peat moss, which retains water, rather than vermiculite. I'm not a fan of vermiculite. Perlite stores absolutely necessary oxygen far better than anything but a couple of items that aren't widely available.
That's the way I went
Vermiculite is better known for its water retention capability. You should already have that with your compost. Wear a breathing mask if you mix vermiculite in your grow medium.