export / import

suppose i have 15gb of users(total space allocated) tablespace so i need to have 15gb on target db or is it the used space that should be same on both prd and dev db's ,Do i need to have same sizes of tablespaces on both source
and target in terms of space allocated ? i already created db,tablespaces thru script on dev box my question is some of the sizes are different (tablespaces) from prod to dev so i need to add datafile to increase the size.

Thanks

All you need is to have the tablespace - same name. Specific size doesn't matter... you just need to allocate enough to accomodate the current used space that's on the source db.