This morning i was puzzled my 50 by 50 land wouldn't spawn any slimes then i did research i found out that. Slimes spawn in any light level but more common in light areas due to more room as there are no other mobs around. They well spawn between 3 bab to 40 bab [bab= blocks above bedrock].

BUT but they will only spawn on 10% of the worlds blocks. therefore you need a massive area for them to spawn. i suggest to do a 60 by 60 square 3 bab. then another 60 by 60 above the ceiling of the first 60 by 60 room which needs a ceiling height of 4+

also Slimes have equal chances to spawn at any size. They do NOT spawn in peaceful mode, however they currently spawn on peaceful for XBox 360 version as of June 26, 2012.
Need any more information just post below thank you