A ver, buscando en Google:
"You can set the size of the virtual ram cart to None, 8kb, 16kb, 32kb or 64kb through the options menu. It is recommended that you always set this to 64kb however because if you later wish to increase the size of the virtual ram cart you will need to reformat it and any existing save data will be lost.
To setup the virtual ram cart for your Sega-CD/Mega-CD game boot the CD or load your CD image as normal. When the Sega-CD/Mega-CD logo screen appears press the A, B or C button to access the advanced menu and then select the memory management features by moving the pointer to "option" or "memory", depending on the bios version being used. Follow the on screen instructions to format your virtual ram cart then start your game as normal."
Es decir...en el Gens:
Opciones -> Tamaño del SRAM de Mega CD -> 64Kb
Cargas un juego de Mega-CD, cuando aparece el logo pulsas A, B o C, y luego en los menús, pues buscas cosas de memory, RAM cartridge, format o initialize (no me acuerdo). Es fácil... No te digo los pasos exactos porque no los recuerdo ni tengo ningún juego de Mega-CD con que probarlo.
Venga, suerte!