IRB laws require that pitches be 100 metres from try-line to try line. However that is the only set distance. The width of the pitch from touch-line to touch-line "should" be as close to 70 metres as "practical", and you'll find that most pitches meant for international play are that 70 metre distance. Try zones may vary from 10 metres in length to 25.

100m plus 10-20m on each end past the try-line

We used to play on the football field (100m) however different teams just used their local field so there was no way to tell.

100

It depends, some pitches are 80 meters long and some are 100 meters long. Twickenham is about 120 meters long tho

100 m I think :/