I presume that you're talking about the RAMSIZE Linker control?
This refers to the internal RAM within the 8051 CPU

I think you probably need to use the XDATA control to specify RAM which is outside the 8051 CPU.

Yes, it can get very confusing with tools like Keil referring to "On-Chip" when they mean "inside the 8051 CPU" and chip makers like Cygnal also referring to "On-Chip" when they mean "inside our chip (but outside its embedded 8051 CPU"

When it comes to downloading your code, presumably the Cygnal utilities must have some way of designating whether it's going to "on-chip" RAM, off-chip RAM, FLASH, PROM or whatever?
Keil has no knowledge of these things; to Keil, all program memory is outside the 8051 CPU.