Hmm, I thought I’d seen an official statement about MP3s being a different size but I can’t find anything now. However, I found this issue already posted by someone else with the only workaround being to use 48KHz.
I personally export as WAVs then use Lame to convert to MP3 manually, but its a command line tool so its a pain to use… You could try this option or another MP3 converter. Mind you, I’ve never checked if the Lame MP3 is the same length or not as I only use MP3 for demos…
Mike.