I have a 10 gallon QT with my 75 gallon tank, but it all depends on how many new fish you want to get at once. I suggest sticking to a low bioload, as the bioload in the QT will fluxuate enough that the biofilter will never reach the peak capacity in your main tank. You may consider the beinifits of multiple QTs over one larger QT, as you can have two diffrent batches of new fish on diffrent timers, or treat a sick fish in one and QT new fish in the other. I have a 5 gallon bucket as my emergency secondary QT.
The easiest way to do it is substrate free, no decorations, no plants, with a sponge or HOB
filter. Lighting needs to be sufficient for observation, but nothing more than that.
Optionally you could add some PVC
sections for reclusive fish to hide in. Optionally you could use other easy to wash or disposeable decorations. If you have some spare floating plant trimmings, it will make some species more comfortable.
Between loads of fish, make sure to continue feeding the tank at least weekly, and clean at the same time as your normal tank. I keep snails in the QT to process the food and make sure I'm remembering to feed enough. (Snail population over time will reflect the food supply.) I use old tank water from my main to refill the QT, so thier water parameters will always be similar. (The sizable amount of driftwood and plants makes the chemistry of the main slightly diffrent than tap.)