We at Arishtam don't keep glass carboy/ demijons or large glass brewing vessels.
1. Glass tends to shatter if not handled with care. It needs to be gently and softly kept on the floor/table otherwise the base will crack.
2. The glass vessels tend to be heavy and a 20 liter vessel filled with beverage weighs as much as a LPG cylinder. So it is not easy to use or move around
3. During brewing/winemaking the hands tend to be wet and handling glass with slippery wet hands is an accident waiting to happen.
4. If the glass breaks, the shards are a health hazard waiting to happen. Also if you pour the beverage in the process. They will go all over the house. If you have pets, kids or elderly, your problem will be compounded manifolds.
Alcohol is a solvent and can leach out chemicals from even ordinary food grade materials. We recommend people to not use PET bottles, tupperware, garbage-bins or any random containers for brewing. If you are using stainless steel, use food grade SS3XX not SS2XX (which is 50% cheaper). When in doubt please refer to FDA, BIS or FSSAI standards on food processing equipment.
What I recommend people to use is HDPE or PPCP. We do carry fermentation buckets that are boiling water proof and will not leach chemicals into your brew.
On the size of the container:
Wine Primary: Use a container that is 1.5-2X your wine must volume. Use of airlock is recommended for very small batches (<5 liters)
Beer primary: 3 inches of headspace is adequate.
Secondary: more than 2-3 inches of headspace is invitation to oxidation. use glass marbles or other fillers to get rid of the headspace.