You can use a large round tip, the larger the tip the larger the balloon.
HTH 
I've never given it much thought either, I would say you could either use a big round tip, start fat and then sort of drag it out a little thinner or you could just cut the end of the bag and go from there too.
I just cut my bag as usual and put in a coupler - to where it comes just barely out of the bag. Then I have a perfect circle. I place that bag straight up and down above the cake make a blob of icing as large as I want the balloon to be then start bringing down decreasing my pressure as I go. I have found that the coupler makes many sizes of balloons and it just works perfectly for me.
I used a large round tip. Yellow cake in my gallery.
Heres a link onhow to do it.
http://www.wilton.com/technique/Balloons
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