Columns can be stored in memory or on disk.
In memory columns will always be stored in memory using
DataMemory. On-disk columns will be stored on disk and in
a page cache. Thus if there is too much data in page cache
they will be spilled to disk, if there is space in page cache or if
the data is hot it will stay in the page cache in memory.
All columns that are part of primary key and all columns that are
part of ordered indexes need to be in-memory. For non-indexed
columns one can select on a column basis or table basis whether
they should be stored in memory or on disk.
The size of a table is dependent on use of VARCHAR columns and
other variable sized columns.
The current size of a table can be discovered using ndbinfo tables.