The FIFA Laws of the Game specify the field dimensions as
Length: min 90 m (100 yds) max 120 m (130 yds)
Width: min 45 m (50 yds) max 90 m (100 yds)
For International Matches
Length: min 100 m (110 yds) max 110 m (120 yds)
Width: min 64 m (70 yds) max 75 m (80 yds)
The length must always be greater than the width.
Many leagues have their own standards for youth matches but they vary greatly from league to league.