One tire on car different size from other three

Thanks for all of the advice. It just boils my blood to have something so simple as matching 4 tires get screwed up. Oh well, guess I’ll just pay the price for the tires, replace the tranny fluid, hope for the best, and thank God tha something worse hadn’t happened.

I recently got my back two tires replaced and I was informed that 6 months ago prior, when I got my front tires replaced that the passenger front was the wrong size! During those 6 months my (93 Chevy Lumina Euro) car has only gotten worse with the problem of chugging constanly when going uphill, and not being able to get power, like it cannot get into gear. Really jerky and chugging. Could this indicate a problem with the transmission due to the incorrect size of that one tire?