The battery wont explode unless the hole that the tube goes into on the battery is clogged or plugged. It would actually be pretty hard to get one to explode even so. The acid would just gurgle out around the little screw in caps and get onto your paint and other stuff and eat it away.
Like Ron said, there should be some area for the tube to go. The length isn't so important, its just making sure that the end of the hose isn't resting against something so that if the battery were over charged or if it went bad and started to vent way way more than normal, the acid would not be all over your moped. If the battery is contained in an enclosed area with electrical connections then the vent hose will also keep that gasses from collecting in the enclosed area and possibly exploding. That would be a greater risk than the battery itself exploding.
When a battery does blow up it is always the gasses inside of the battery that are burnt. The gasses are what is combustable (thats why there are always no smoking signs near battery racks and why chargers carry warnings about being in a well ventilated areas and also warn against not smoking around the battery)
Rember to keep the water level where it is supposed to be. That will reduce the amount of gassing in the battery and also keep the cells in better shape so that you wont have to worry about a short in the battery. If you ever notice a swelling on a battery, replace it!!