Using fbset, you can program video modes (see the framebuffer documentation in the kernel tree for details). However, there are some modes built into most drivers that you can specify at boot time: typical mode names are 640x480, 800x600 and 1024x768. Some cards can also autodetect the mode you enable in AmigaOS and use it.
As of kernel 2.1.124, here is a fairly comprehensive list of supported modes. "bpp" stands for "bits per pixel": it is the color depth. All modes are non-interlaced unless otherwise specified.
CLGen (Piccolo; Piccolo SD64; Picasso II; Picasso IV; Spectrum):
Autodetect: Use mode set in AmigaOS (with --keep-video option to Amiboot)
640x480: 31.25 kHz, 60 Hz, 25 MHz PixClock (8 bpp)
1024x768: 55.8 kHz, 70 Hz, 80 MHz PixClock (8 bpp)
Cybervision 64 (original and 3D):
640x480-8
800x600-8
1024x768-8
1152x886-8
1280x1024-8
1600x1200-8
800x600-16
Retina Z3:
640x480: 8 bpp
800x600: 8 bpp
1024x768i: 8 bpp, interlaced
640x480-16: 16 bpp
640x480-24: 24 bpp