Make it as long as the story demands - worst case scenario is if it's too big, it can be split into parts (e.g. A Song of Ice and Fire).
There are numerous examples of series with varying book length.
Raymond E Feists Riftwar is a good start - Magician is a substantial book by itself, the following two (Silverthorn and A Darkness at Sethanon) noticeably less.
Wiki puts the page count for that trilogy as follows:
Darkness at Sethanon: 425
Plus there are the likes of Harry Potter or Obernewtyn etc where the first books are smaller then the following ones.
There is no 'rule', nor anything to really be gained, by making your books match each other in length. The length should be determined by the story.