As you have discovered, it is all too easy for a newbie to "upgrade" their FSD and have that actually be a "downgrade".
For ship modules such as FSDs, the descriptor is a number and a letter. For example, "3C". The number (3) is the size. The letter (C) is the rating, or how good that module is compared to other modules of the same size.
For FSDs, you should never, ever accept a reduction in size if you are trying to increase jump range. The reduced weight never compensates for the lack of power a smaller drive offers. So a smaller, better module is never going to have the same jump range as the maximum sized drive your ship can take, even if it is lower rated. It's like trying to install a motorcycle engine into a tank; it just can't do it, the tank is simply too big and heavy for that engine. In other words, buying a 2B or 2A drive to replace the 3C drive is not going to give you better range. If your ship has a 3C drive and you want to upgrade it for better range, you need to buy a 3B or 3A drive, or a size 4 drive if your ship can take a size 4.