But now, when building, and error message (#249) from the compiler tells me that the DATA segment is too large. This seems as if the whole RAM was not declared, and this is the case.
you have only 128 bytes of DATA, the top 128 are IDATA.

In the linker's option dialog box, the acceptable maximum value for RAMSIZE is 256 bytes. How could I specify that the RAM size is 1Kbyte
You can't, there is only 256 bytes. ERAM is XDATA it just happens to be local to the chip, the access to XDATA or EDATA is controlled by the AUXR, the instruction is the same (movx). PDATA is based on loading R2 with the high address and the EDATA does not work that way (if it did P2 would not be useable for other things). See my previous post re AUXR load.

Have fun,

Erik