@Heather13... and f you are talking about carbonated beverages in general, the reason my my doc gave to lay off them is because drinking them contribute to excess gas in your stomach, which is not your friend
...and NOT because of stretching your stomach. One would have to have their stomachs filled beyond capacity (with liquids OR solids) for extended periods of time to do that. And even then, it is highly, highly unlikely a sleeved stomach would stretch to pre-surgery size. Not even close. If under some crazy circumstances one was able to stretch their stomach close to even HALF thier original size (given a sleeved stomach is 20% of original size), that person would have waaaaay bigger problems to worry about than a bigger stomach.
With that said, i was drinking carbonated beverages around 2-3 months post op, and still do, almost every day. HOWEVER, i am drinking sparkling water (i.e., Perrier or similar, with no sugar nor flavourings) and NOT pop/ soda nor beer. Too many unnecessary calories for me, personally. I have hade diet coke a handful of times in the past year though (and only once i was done trying to lose weight)...but is is def the exception and not the rule.