Mob Spawner Dimensions

Legend
S: Spawner
X: Mobs spawn within (but not at) this boundary
@: Too many mobs within (but not at) this boundary prevents more spawns
0-F: Measurements

 @@@@@@@@@@@@@@@@@@@
0@        3        @
1@        2        @
2@        1        @
3@   XXXXXXXXXX    @
4@   X    4   X    @
5@   X    3   X    @
6@   X    2   X    @
7@   X    1   X    @  N
S@321X4321S123X1234@ W+E
8@   X    1   X    @  S
9@   X    2   X    @
A@   X    3   X    @
B@   XXXXXXXXXX    @
C@        1        @
D@        2        @
E@        3        @
F@        4        @
 @@@@@@@@@@@@@@@@@@@
  01234567S89ABCDEF

Profile view:

-6- Ceiling
-5- Air Block
-4- Air Block
-3- Spawner
-2- Air Block
-1- Water Flow
-0- Floor

(Ground, water flow, four air blocks above the water, ceiling.)

Placing a block on the spawner itself prevent mobs from spawning on top of it.