If you already have retropie installed just plug it in and it will work.
Then use
sudo raspi-config
to expand your filesystem to utilise all of the space on your card.
If you are on windows and it only shows your SD card as 50mb that is because the rest of the card is formatted as exfat which windows can’t read so to windows the rest of your SD card is essentially invisible. Exfat is a format used by linux. (As opposed to fat32 used by windows)
If your card by default has 50mb without you doing anything to it with win32diskimager likelihood is that its corrupted and its time for a new SD card (that is what happened to me once)
See this set of YouTube videos to get yourself on your feet from a fresh install: