I think it would be fun if you added a harder difficulty where the barista max size is even bigger. The catch is, you can no longer increase her size by simply spending money. Instead, you have to keep her at max size for a certain amount of time to trigger an event that will cause the max size to increase. 

I like this idea. I thought it was too simple to just be able purchase bigger sizes. Perhaps a two-step system. The purchase allows her to get bigger but forces the player to push her past her previous maximum size to unlock the added capacity.

This would also make the upgrade that allows her to be at max size longer to be needed. I not once needed that upgrade unless I was doing the milky mode on hard.

I think that's an awesome idea and I was thinking something similar.

Agreed; I had thought of something similar as well. Make both max size and production increase based on how you milk the barista.

More than 95% full = slow max size increase, until milked down under 75%

Milked from at least 80% full down to less than 10% (drain the tanks to near empty) = small production increase

For added difficulty and strategy, add an "overstretched" debuff bar - increases as her max size does; decreases when not near max size. High debuff causes more happiness (smiles%) lost. ..... Would prevent you from trying to race to max size.... might make it too difficult though, esp late-game hard milky or chaos milky modes where she fills up almost instantly XD