The Atari VCS has an open M.2 slot on the motherboard, which allows you to add an M.2 SSD for additional storage.
M.2 compatibility -- The Atari VCS is compatible with M.2 SATA SSD drives. The SATA format has both a B and an M key -- the two notches at the bottom in the picture below. The NVMe format, which is incompatible with the Atari VCS, has only one notch. There is no known restriction on storage capacity.
Time and effort -- Installing an M.2 SSD in your Atari VCS is not an overly difficult task but does require some care to ensure you do not damage your VCS during the installation process. Both written and video installation guides are available in the links below to guide you through the process.
M.2 SSD Installation Guide: https://atarivcs.com/content/SSD-Installation-Guide.pdf
M.2 SSD Installation Guide (Video): https://youtu.be/rhc6g_p28TQ