It's very, very rare that a supporter import file will be too large to import in one chunk. It has happened before though.
In the previous case, the import file in question was 31.5 MB and had 479900 rows to import. The import timed out around 60% (no records imported, just a time-out prior to proceeding to Step 2).
The solution is very simple. Break up very, very large imports into multiple files. In the example above, the file was split into two (about a quarter million records in each file, of just over 15 MB each). The imports processed just fine.