unter
https://www.kernel.org/doc/Documentation/m68k/kernel-options.txt finde ich:
4.1.1) Video Mode
-----------------
This sub-option may be any of the predefined video modes, as listed
in atari/atafb.c in the Linux/m68k source tree. The kernel will
activate the given video mode at boot time and make it the default
mode, if the hardware allows. Currently defined names are:
- stlow : 320x200x4
- stmid, default5 : 640x200x2
- sthigh, default4: 640x400x1
- ttlow : 320x480x8, TT only
- ttmid, default1 : 640x480x4, TT only
- tthigh, default2: 1280x960x1, TT only
- vga2 : 640x480x1, Falcon only
- vga4 : 640x480x2, Falcon only
- vga16, default3 : 640x480x4, Falcon only
- vga256 : 640x480x8, Falcon only
- falh2 : 896x608x1, Falcon only
- falh16 : 896x608x4, Falcon only
Den Rest der bootargs hast du wie auf der netinst CD gelassen? Lass probeweise das -S am Anfang weg, dann kommt der Kernel nicht ins ST sondern ins TT RAM.