The size of the picth is not evenly in every field. There are maximum and minimun size.
Law 1 told us about it:
The touchline must be longer than the goal-line.
minimum 90m (100yds)
maximum 120m (130yds)
minimum 45m (50yds)
maximum 90m (100yds)
Dimensions for international matches
minimum 100m (100yds)
maximum 110m (120yds)
minimum 64m (70yds)
maximum 75m (80yds)
Competitions may determine the length of the goal-line and touchline within the above dimensions.
There is no rule about that the pitch has to maintain some aspect. in fact a field can be 90m X 45m or 90m X 89m, but the last one would be a very strange field.