Where is the largest sun dial ? And how big is it ?

According to this site the Jantar Mantar Observatory. The observatory is located outside and consists of 18 separate structures that are used to tell time, day of year, sun azimuth, sun zenith and other sun-earth variables. It contains the worlds largest sun-dial that is over 27 metres tall. We checked the sun-dial with our own watches and it was an exact match. A facinating place. World's largest sun dial

Frankvan,

Since your watches exactly matched the sundial, that should mean you were in the east-west center of that time zone, 'cause sun time matches standard time only in such locations.

Sun time: noon is when the Sun reaches its highest point in the sky.

Standard time: noon occurs at a time agreed upon for a whole time zone, whether or not the Sun it at its highest point.

Was the Observatory in the middle of its time zone?

The person whose report is given in the link I provided is Andrew Costello, and the location is apparently in Jaipur, India. I would assume that the accuracy he found was probably agreement between their watches and the number of minutes before or after the hour indicated - with perhaps a little poetic license.

after you set up your sundial, aiming it true north with the gnomon (shadow casting object)pointing at polaris and the angle of the gnomon equal to your latitude, there are three steps necessary to convert sun time to clock time. The sundial position then should never be changed.
1. equation of time from a chart. this represents the changing speed of the earth as it travels around the sun. it could be about as much as 16 1/2 minutes plus or minus.
2. longitude factor. you figure plus or minus 4 minutes for every degree you are east or west of the center of your time zone.
3. add an hour if you are on day light saving time.

it's been along time since i've made a sundial, but i think that i am correct.

tv

My school was in the close of Salisbury Cathedral. As part of a science lesson, we marked out a sundial on the main quadrangle, using the shadow of the cathedral spire (404 feet). It was only good for about three hours a day, but it was certainly big!

My sundial has a "chart" on the face of it showing how many minutes to add/subtract according to the day and month.

And once I get it pointed accurately, it keeps amazing time!