The size of a tornado is determined by the width of the area over which it produces damage. This is not to be confused with the tornado's intensity.
The damage severity in a tornado is determined primarily by wind speed. For example, a tornado with peak winds of 100 mph, even if it is very large, is not going to obliterate well built houses, thought it will cause severe roof damage. By contrast, even a relatively small tornado with peak winds of 250 mph can completely destroy just about any structure.
The damage is surveyed and where damage boundaries are is noted. This is the used to show how wide the tornado is. Note that the size is not a factor in how the tornado is rated but how intense the damage is.
There is no particular size, as tornado ratings are based on the severity of the damage caused, not the size of the tornado. That said, F0 tornadoes are typically small. Most are less than 100 yards wide.
Meteorologists and engineers look at the aftermath to determine the intensity of the winds. It is possible to get a rough idea of how strong a tornado is by how fast its rotating.
The average tornado is 50 yards wide and has wind in the range of 70 to 90 mph.
By how strong the tornado is. Even though strong tornadoes tend to be larger how strong a tornado is does not determine its size. Relatively weak tornadoes have been very large and extremely strong tornadoes have been relatively small.
The size of a tornado, or its width is determined by how wide the area of damage is. The rating of a tornado, which is not dependent on size (though there is some correlation) is based on the severity of the damage caused.
Ted Fujita discovered the Fujita Scale to determine how much strength a tornado has based on the damage it produces.
A tornado shelter typically ranges in size between that of a large closet and a small office.
That is impossible to determine.
The damage severity in a tornado is determined primarily by wind speed. For example, a tornado with peak winds of 100 mph, even if it is very large, is not going to obliterate well built houses, thought it will cause severe roof damage. By contrast, even a relatively small tornado with peak winds of 250 mph can completely destroy just about any structure.
The damage is surveyed and where damage boundaries are is noted. This is the used to show how wide the tornado is. Note that the size is not a factor in how the tornado is rated but how intense the damage is.
No machine is used. The intensity of a tornado is determined by a visual survey of the damage.
The Xenia tornado of 1974 was about half a mile wide.
In most cases the wind speed of a tornado is estimated based on the severity of the damage it causes.
50 yards
It is impossible to determine where or when the next tornado will be. The potential for a tornado in a specific place can't be predicted much more than a few minutes ahead of time. For a general region (at least the size of a state), the potential for tornadoes can be seen hours, or sometimes days ahead of time, but it is impossible to tell where or when individual tornadoes will hit.