I had exactly the same issue as dirtystopout, basically it seemed Logic was calculating the size of a PCM bounce, regardless of what I had ticked in the bounce window. I got round it by changing the settings under PCM to the lowest resolution and sample rate so that Logic calculated a file size smaller than the maximum for a WAV (4GB), then bounced it with just MP3 ticked.
This feels like a bug of some sort, and I've never had/noticed it happen before - could it be Yosemite-induced?
FWIW: iMac Intel Core i5 / 3.1 GHz / 12GB RAM / OSX 10.10.1 / Logic 9.1.8