According to 3DO development documents:

A minimum 3DO system includes 2 MB of DRAM and 1 MB of VRAM for a total of 3 MB of RAM. Optional memory configurations can, for this version of the hardware, go up to a maximum of 16 MB of RAM: 1 MB of VRAM and 15 MB of DRAM, or 2 MB of VRAM and 14 MB of DRAM.

Memory address 0x0330_0004, a MADAM register, is used to indicate the 3DO's memory configuration. While looking over the FZ-10 ROM in Ghidra the function for calculating the size of DRAM (or starting address of VRAM) was found at address 0x03000518. Here is a decompiled version of that code. The MADAM register can be written to, and is a few times by a few routines, but it is unclear if and how that impacts the system. Perhaps it informs MADAM or other components of the memory configuration. A different function (0x0300_02C4) explicitly tests the RAM.

dram_size.c

int dram_size(void)
 
{
  uint bank1_size;
  uint mem_cfg;
  uint bank2_size;
  int dram_size;
 
  mem_cfg = read_volatile_4(DAT_03300004);
  bank2_size = (mem_cfg & 0x60)>> 5;
  if (1 <bank2_size) {
    bank2_size = 4;
  }
  bank1_size = (mem_cfg & 0x18)>> 3;
  if (bank1_size <2) {
    bank2_size += bank1_size;
  }
  if (bank1_size == 2) {
    bank2_size += 4;
  }
  if (2 <bank1_size) {
    bank2_size = 0x10;
  }
  dram_size = bank2_size * 0x100000;
  if (2 <bank1_size) {
    dram_size += (mem_cfg & 7) * -0x100000;
  }
  return dram_size;
}

Only bits 6-0 are used. For convenience here is a table of all supported configurations including the available memory as reported by the AvailMem function.

• Given there is 48MB of address space between 0x0000_0000 and the ROM at 0x0300_0000 it might be possible to hack the ROM to allow for 16MB + 16MB of DRAM and 16MB of VRAM.
• FreeDO/4DO was hard coded to support the retail model's memory configuration. In the least it should be possible to enhance Opera to support the official permutations.
• Find out what writing to 0x0330_0004 does.