If you create a datafile, you have to take in account the datafile header.

So,
if you want to make a datafile that containts 512Mb,
create a datafile with size 512M + 1block.

eg, if you have a 4K block db,
alter tablespace .... add datafile ... size 524292K;

524292 = ( 512 * 1024 ) + 4

Hope this helps
Gert