Very big files

I'm trying to convert a book (a dictionary) in mobi format to epub format. The file is very large about 6MB. After 30 minutes the progress is 47%. Is there a size limitation to convert? Why cannot Calibre convert this file? Some time ago I tried with another dictionary and the same thing happened.

Thanks,

P.S. I've a Sony Reader. I cannot add new dictionaries. So, I'm looking for a dictionary (German - Spanish) in epub format.