I've seen two types of Jelly fishes here in Bahrain. The type you see in the images below (don't know what it's called) and another one locally referred to as "El-Dour". Now from what people tell me that bare skin contact with the first type causes some itchiness that goes away quickly and may be a bit annoying. Besides this type's length could go up to 1ft or just a margin above.
However, with the "El-Dour", contact will seem like an electrical shock which is quit painful and may cause some swelling in the area of contact.
As a diver, I learned to cure this immediately with Vinegar. Vinegar neutralises the area of contact. It's very important not to scratch the affected area (poisonous cells from the jelly fish may be spread around), NOT to pour fresh water on it and not go under direct sunlight for a while until the pain goes away. In severe cases it's recommended to apply a Hot pack. Hot pack is an essential part of the diving medical kit which heats up to 45C or 50C. The idea in applying heat is a universal solution to slowing down poison in general and not just for jelly fish stings. The analogy behind that is that poison is composed of proteins. And the effect of such protein wears off by cooking it (Applying heat to it).
The average "El-Dour" jelly's length could be around 2ft to 4ft. However, I have seen up to 10ft long here in Bahrain. Long tentacles! I thank god our jelly stings are not fatal or serious. Some countries like Australia have deadly ones such as the box jelly fish and the blue bottle.