Officially, to the best of my knowledge, no Neo-Geo game has used a sample ROM greater than 16MB, and ADPCM-A and ADPCM-B were often merged into the same ROM in most if not all Neo-Geo games late in the hardware's commercial lifespan.
It has been proven by Blastar (
https://blastar.citavia.de/) that the YM2610 can address *at least* 32MB (16MB for ADPCM-A, 16MB for ADPCM-B).
Basically, a 'steroid boost' of the original 'pre-PCM' configuration (2MB for ADPCM-A, 2MB for ADPCM-B) which many early games such as Magician Lord and NAM-1975 used.
I'm not sure if the Neo-Geo itself can go above 32MB (Blastar hasn't been able to go above 16MB on actual hardware), but I would imagine a custom PCB could use 3 or more 16MB ROMs with a mapper.
The YM2610 does have the advantage of two different ADPCM types, so bank-switching might be easier as you could dedicate certain banks to ADPCM-A and others to ADPCM-B.
It might be tougher with the Y8950 and YM2608.
With only one channel you have to consider your use cases carefully.
The extra ROM space could perhaps be used most advantageously for streaming music (leaving SFX and certain songs to the FM [+ PSG/rhythm in the 2608]) and extended amounts of sampled speech (ideally in cut-scenes and in-game situations where a sample won't be cut off).
Thank you for your response!