Dmeid!
The size of a Pdox table file depends on number of records and size of each record. When files start becoming "too" large, their reaction starts to slow down. In a relational database, such as Pdox is, you can often break down one large table into many small ones.
I don't know how to answer to your second question, data security and integrity. (In Pdox these two attributes have different meanings).
Have a look at the forum Corel: Paradox here at Tek-Tips.
Regards mjt39