The are usually 10 or 20 gallons. Kinda depends on the size fish you eventually plan to buy. You need the water to be moving, so a small power head and/or filter works for that. Some people keep a small filter, or sponge, in their sump to add when they use their QT. that helps by adding beneficial bacteria right away. If temps would fluctuate a lot in your residence, you might need a heater. Go bare bottom and buy a piece or two of big PVC
for hiding (places like Home Depot have this).
Places like Petco often have dollar a gallon deals for tanks. Filters aren't much. PVC
is inexpensive. Heater, if needed, isn't much. When you figure the cost of the fish you are going to eventually buy and/or try to save, this really is an inexpensive piece of insurance to own. Treat fish in this now. Get the DT clear of Ich by leaving fallow. Put healthy fish back in. Then, QT future additions and treat if needed. Add those healthy fish to a healthy aquarium. Your DT should stay disease-free.