While I don't have a particularly large file, many of the users that frequent the Reunion Forums work away with huge files.
I suspect that the biggest issue will be exporting/importing your GEDCOM and preserving your sources. Again, this is something that you can get help with on the Reunion Talk forum. I have found their help and responsiveness invaluable.
I gave up on FTMM a long time ago & switched to Reunion. It's the best thing I've done in a long time.
Best of luck.