What you are referring to is Framebuffer resolution. When you first installed slackware you were probably using the noveau driver which was controlling your frambuffer resolution, unfortunately the nvidia binary driver doesn't have framebuffer support, you will have to use the unaccelerated vesafb, however a lot of modern video cards support higher resolution modes with the vesa driver. To check and see what your card supports you need to install two packages from
www.slackbuilds.org, install them in this order.
libx86emu
hwinfo
Once these two packages are installed you need to run hwinfo like this...
hwinfo --vbe
Towards the bottom of the output you should see information like this
02: None 00.0: 11001 VESA Framebuffer
[Created at bios.459]
Unique ID: rdCR.BfO5bYYtG87
Hardware Class: framebuffer
Model: "NVIDIA GF104 Board - 10410001"
Vendor: "NVIDIA Corporation"
Device: "GF104 Board - 10410001"
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
Mode 0x0303: 800x600 (+800), 8 bits
Mode 0x0305: 1024x768 (+1024), 8 bits
Mode 0x0307: 1280x1024 (+1280), 8 bits
Mode 0x030e: 320x200 (+640), 16 bits
Mode 0x030f: 320x200 (+1280), 24 bits
Mode 0x0311: 640x480 (+1280), 16 bits
Mode 0x0312: 640x480 (+2560), 24 bits
Mode 0x0314: 800x600 (+1600), 16 bits
Mode 0x0315: 800x600 (+3200), 24 bits
Mode 0x0317: 1024x768 (+2048), 16 bits
Mode 0x0318: 1024x768 (+4096), 24 bits
Mode 0x031a: 1280x1024 (+2560), 16 bits
Mode 0x031b: 1280x1024 (+5120), 24 bits
Mode 0x0330: 320x200 (+320), 8 bits
Mode 0x0331: 320x400 (+320), 8 bits
Mode 0x0332: 320x400 (+640), 16 bits
Mode 0x0333: 320x400 (+1280), 24 bits
Mode 0x0334: 320x240 (+320), 8 bits
Mode 0x0335: 320x240 (+640), 16 bits
Mode 0x0336: 320x240 (+1280), 24 bits
Mode 0x033d: 640x400 (+1280), 16 bits
Mode 0x033e: 640x400 (+2560), 24 bits
Mode 0x0345: 1600x1200 (+1600), 8 bits
Mode 0x0346: 1600x1200 (+3200), 16 bits
Mode 0x034a: 1600x1200 (+6400), 24 bits
Mode 0x0360: 1280x800 (+1280), 8 bits
Mode 0x0361: 1280x800 (+5120), 24 bits
Config Status: cfg=new, avail=yes, need=no, active=unknown
03: None 00.0: 10002 LCD Monitor
[Created at monitor.95]
Unique ID: rdCR.xY13vwLsp9C
Hardware Class: monitor
Model: "HF289H"
Vendor: HSD
Device: eisa 0x190b "HF289H"
Serial ID: "851DHM05A0984"
Resolution: 720x400@70Hz
Resolution: 640x480@60Hz
Resolution: 640x480@67Hz
Resolution: 640x480@72Hz
Resolution: 640x480@75Hz
Resolution: 800x600@56Hz
Resolution: 800x600@60Hz
Resolution: 800x600@72Hz
Resolution: 800x600@75Hz
Resolution: 832x624@75Hz
Resolution: 1024x768@60Hz
Resolution: 1024x768@70Hz
Resolution: 1024x768@75Hz
Resolution: 1280x1024@75Hz
Resolution: 1600x1200@60Hz
Resolution: 1920x1200@60Hz
Size: 610x350 mm
Detailed Timings #0:
Resolution: 1920x1200
Horizontal: 1920 1968 2000 2080 (+48 +80 +160) -hsync
Vertical: 1200 1203 1209 1235 (+3 +9 +35) +vsync
Frequencies: 154.00 MHz, 74.04 kHz, 59.95 Hz
Driver Info #0:
Max. Resolution: 1920x1200
Vert. Sync Range: 56-75 Hz
Hor. Sync Range: 24-80 kHz
Bandwidth: 154 MHz
Config Status: cfg=new, avail=yes, need=no, active=unknown
The text in red is the information your looking for, my monitor has a native resolution of 1920x1200, however as you can see the max frambuffer resolution I can use is 1600x1200@24bits. Your card however may support different resolutions, my older 8800GT card actually supported 1920x1200 but my current GTX-460 Cant 
, unfortunatly there is nothing you can do about this. Now to use the selected resolution edit your /etc/lilo.conf file and change your vga setting to the resolution you want, for instance if I wanted to use 1600x1200 I would change it like this
vga = 791
to
vga = 0x034a (Or what ever resolution your card supports and you want to use)
save the file and rerun lilo command as root "lilo"
and upon reboot you should be good to go.
