The biggest single tank you can find the better. You want the sewage to have more time in the tank. More time to settle apart so the effluent will be as clear as possible. A home septic tank is sized based on gallons per day and at 60 gallons per person, 2 people per bedroom, makes 120 gallons per day per bedroom, a 3 bedroom house uses a 1000 gallon tank to handle that 360 gallons per day. If your waste flow are more like 36 gallons per day then your septic tank can be 100 gallons or two barrels. You would be much better off using an old oil tank which is in the 250-300 gallon range.
The septic tank only provides primary treatment and what comes out will still be in need of secondary treatment by somehow aerating it. A drainfield will work for that if the water table is low enough.
Honestly, a real, concrete, 1000 gallon, septic tank with all the baffles and access ports is only 600$ in these parts. The delivery trucks will set it in your hole. Getting that pumped will be easier than a couple of barrels and will work better too.