An MDB file cannot exceed 2 Gb.
In normal usage, your application should be split into a front-end
(containing the queries, forms, reports, macros and modules), linked to a
back-end (containing the tables and relationships). That gives you 2 Gb for
the application (the front-end), and 2 Gb for the back-end. Of course, if
you're coming close to that limit, you might want to consider moving your
data to SQL Server or Oracle.