I'd guess about 1 MB order of magnitude. That would be a butt-load even for samples.

1 MB would be enough for 45 seconds audio at 8 bit PCM @22 kHz. If they're half competent, they could use ADPCM or much better (and cheaper to decode too!) vector quantization.

With VQ, you could go even down to ~1 bits (~6 minutes of audio per megabyte) per sample while maintaining good audio quality. Decoding is very simple, so 6502 would have plenty of oomph to do that.

the poster mentioned it was for greeting cards, I remember their used to be a lot of greeting cards that would play a recording when you opened them. I bet they loaded low-quality samples into the ROM for that.

The 6502 can only address 64K, so I'd assume 32K of ROM at 8000-FFFF with 128 (not 100) bytes of RAM mapped at both 80-FF (zero page) and 180-1FF (for the stack) and 128 MMIO bytes at 00-7F. It depends on how much they've modified the base 6502 design, though; the above is for "not at all".

For a $0.06 part? Uh, clearly not TB...