Oracle datafile size

What is the limitation for oracle datafile size?

It depends on 2 factors:
i. OS and ii. Database block size (DB_BLOCK_SIZE) parameter.
In 32 bit OS, You can create datafile upto 2GB to 4GB.
Following is the impact of DB_BLOCK_SIZE parameter on datafile size limitation:
For smallfile tablespace, single datafile can hold upto 2^22 or 4 MB or 4 million blocks, it means:
with DB_BLOCK_SIZE=4k, you can have max file size= 4k*4MB =16GB
with DB_BLOCK_SIZE=8k, you can have max file size= 8k*4MB =32GB
with DB_BLOCK_SIZE=16k, you can have max file size= 16k*4MB =64GB and so on..

For Bigfile tablespace(10g feature), a single data file can hold upto 2^32 or 4GB or 4 billion blocks, it means:
with DB_BLOCK_SIZE=4k, you can have max file size= 4k*4GB =16TB
with DB_BLOCK_SIZE=8k, you can have max file size= 8k*4GB =32TB
with DB_BLOCK_SIZE=16k, you can have max file size= 16k*4GB =64TB and so on..
Other Limits you can find in following Oracle Document:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14237/limits002.htm#i287915