The -m 256 option should be not necessary anymore. The minimal RAM amount for the emulated SPARCStation-5 is now 32 MiB. So OBP should understand now the memory sizes of 32, 64, 96, 128, 160,192, 224 and 256 MiB. When not specified, the default value of 128 MiB is taken.
The support of RAM size <256MiB is important for SunOS 4.1.4 (aka Solaris 1.1.2) because its installer crashes under qemu when started with "-m 256" option.
Will update the how-to shortly.