It depends on how large a db file phpmyadmin will accept without timing out. ;) , your host may be willing to adjust this (I "think" its not actually the php.ini upload limit its set it my.ini although I'd have to check that)
Myself, if its over about 5mb I would not try it and wait ages for it to just timeout. :)
See my reply above (post 7) , I'd suggest using BigDump (although sometimes you can get some strange issues) if possible as its a good tool to use and very straightforward once you have used it once, its easy.
If I'm using localhost I just use the command line as its fast and always works. :)