You need to pass an option at the boot prompt or edit lilo.conf accordingly. Here is from man lilo.conf:
This specifies the VGA text mode that should be selected when
booting. It may be specified as a global option. The following
values are recognized (case is ignored):
normal: select normal 80x25 text mode.
extended (or ext): select 80x50 text mode.
ask: stop and ask for user input (at boot time).
<number>: use the corresponding text mode. A list of available
modes can be obtained by booting with vga=ask and pressing
If this variable is omitted, the VGA mode setting contained in
the kernel image is used. (And that is set at compile time using
the SVGA_MODE variable in the kernel Makefile, and can later be
changed with the rdev(8) program.)
Press ESC at the boot menu and type
and you'll get the available options.