what does it mean that PsoC 3 have a stack on 256 bytes?
That is literally true. Stack in KEIL-compiler is only used for the return-address of function calls and interrupts. The usual C-lanugage stack is emulated.
Want an advice? Use a PSoC5, it is a quite more modern design.
Ok. what is the size of PSoC 4/5 stack?
As a maximum you may define all the sram in a PSoC4/5 as stack. So there is the only limitation the 32k or whatever your PSoC has got. Quite comfortable size to play with.
Can you tell us what your problem exactly is, or what you want the PSoC to perform?
I need to argue what the actual stack size of the PSoC 3 is.
I have read that it is 128b for a regular 8051.
It has to do with the limitations in the PSoC 3 interrupt routines.
The interrupt controller has an 8 level stack. Have a look into the technical ref manual.