If you use text mode, as I do, then you will need more than 80 characters by 25 lines to work efficiently. I have found it is always possible to have at least 1024×768 resolution and usually even higher.

The best way is to probe the hardware using the SuSE hwinfo tool, available for Gentoo through emerge:

emerge -va hwinfo

Then have it probe the hardware:

hwinfo --framebuffer

The output will look something like this:

$ sudo hwinfo --framebuffer
02: None 00.0: 11001 VESA Framebuffer
  [Created at bios.459]
  Unique ID: rdCR.VWINrtDIJN2
  Hardware Class: framebuffer
  Model: "NVIDIA G86 Board - NV_NB8M"
  Vendor: "NVIDIA Corporation"
  Device: "G86 Board - NV_NB8M"
  SubVendor: "NVIDIA"
  SubDevice:
  Revision: "Chip Rev"
  Memory Size: 14 MB
  Memory Range: 0xd5000000-0xd5dfffff (rw)
  Mode 0x0300: 640x400 (+640), 8 bits
  Mode 0x0301: 640x480 (+640), 8 bits
[snip]
  Mode 0x0364: 1440x900 (+1440), 8 bits
  Mode 0x0365: 1440x900 (+5760), 24 bits

Pick the size you want and put “vga=” followed by the mode on your kernel line in /boot/grub/menu.lst, e.g.,

kernel  /vmlinuz root=/dev/sda7 vga=0x0365 resume=/dev/sda6

You can just pick a standard VGA code (or even a common non-standard one and hope it works). Refer to the VESA_BIOS_Extensions Wikipedia page or the table below.