I never really exchanged stuff until i had kids either. I think instead of you being the tacky one, the person that ignores your size suggestions and doesnt' give a gift receipt is the tacky one. If they ask about it and i hardly ever had anyone do that, just thank them so much and say how cute it was. If they specifically ask if it fit her, you could say "she is just growng so fast" and change the subject to talk about her crawlign or what ever.
Here is some advice though, i found that people would buy clothes on clearance and that when i tried to return it i would NOT get enough back to go buy fall clothes, Especially at Kohls. I had someone buy a sweet little white sweater from Carters and when i tried to return it with no reciept they wanted togive me 98 cents. So it was a better value to save the larger size summer clothes until next year. You could try to consign/resale some stuff if you have a place close to you As a place to get rid of what you don't want and a place to pick up what you need for fall.
People just don't think sometimes, and they really don't care about what she needs its their right when giving a gift to just pick out what they think is cute.