In the past I used to have perfect screen resolution with a Slackware guest in virtualbox, but for a while now (Slackware 14.2, virtualbox 4.3.24), compiling Guest Additions fails. It ends with:

/tmp/vbox.0/lnkops.c:58:23: note: (near initialization for 'sf_lnk_iops.put_link') 
cc1: some warnings being treated as errors 
scripts/Makefile.build:258: recipe for target '/tmp/vbox.0/lnkops.o' failed 
make[2]: *** [/tmp/vbox.0/lnkops.o] Error 1 
Makefile:1384: recipe for target '_module_/tmp/vbox.0' failed 
make[1]: *** [_module_/tmp/vbox.0] Error 2 
/tmp/vbox.0/Makefile.include.footer:79: recipe for target 'vboxsf' failed 
make: *** [vboxsf] Error 2 
Creating user for the Guest Additions. 
Creating udev rule for the Guest Additions kernel module. 

Unsurprisingly, when I boot the guest, modprobe fails as well:

Starting the VirtualBox Guest Additions ...fail! 
(modprobe vboxsf failed) 
Starting VirtualBox Guest Addition service ...done.  

Therefore I’m stuck on a tiny screen resolution. After almost two hours of searching, I had to conclude that no official solution exists. I did find a less perfect experimental solution:

https://www.whonix.org/wiki/Higher_Screen_Resolution

Following the instructions there gives me a maximum resolution of 1600x1200. Not quite the size of my screen, but I think I can live with it. (If the site is down or page is gone, I saved the relevant bit here).

Notes:

(1) xrandr gives now:

Screen 0: minimum 320 x 200, current 1600 x 1200, maximum 1600 x 1200 
default connected 1600x1200+0+0 0mm x 0mm 
   1600x1200     86.00*  
   1280x1024     86.00   
   1152x864      76.00   
   1024x768      85.00   
   800x600       86.00   
   640x480       86.00   
   640x400       86.00   
   320x200        0.00   

(2) I had to uninstall the Guest Additions, which turned out to be unintuitive. Google gave a somewhat unhelpful page: Re: How to uninstall Guest Additions in Ubuntu guest? until someone added to it:

Postby TheBigKahoona » 27. May 2011, 13:58 
for everyone how ends up here by googleing this matter: 
/opt/[VboxAddonsFolder]/uninstall.sh 

That was nice.