The Premier League introduced rules two years ago to standardise pitches at 100m by 68m. The only pitches that are allowed to be smaller are those in older grounds, like White Hart Lane and Stamford Bridge, where there isn't room to extend the pitch outwards.
Because Wembley does have the space to meet the Premier League's minimum requirement for a pitch, Spurs won't be allowed to make the dimensions any smaller, although they could request it is reduced from 69m to 68m, per the league's rules.
According to the Times, it was made clear to Tottenham at a meeting of the Premier League's 20 chairman that they would not be allowed special dispensation to change the size of the pitch.
That could be an issue for Spurs as they seek to improve upon their second-placed finish last season. Pochettino's men secured a record high finish and points haul, but fell short in the title race behind eventual champions Chelsea.
When it is built, the pitch at Tottenham's new stadium will also have to conform to the pitch size rules.