The MacBook is 13.3" and has a resolution of 2560x1600.
VirtualBox 5 adds the option "Use Unscaled HiDPI Output" under Screen>Screen in the virtual machine's settings.
With this off (as with VirtualBox 4), the resolution reported to Linux guests is 1440x900. This is the resolution OS X simulates, but this causes blurriness in Linux.
With this on, the resolution reported to the Linux guest (NixOS) is 2880x1800, bigger than my native resolution, instead being that of a 15-inch MacBook.
xrandr reports the following on the guest:
Screen 0: minimum 64 x 64, current 2880 x 1800, maximum 16384 x 16384
VGA-0 connected primary 2880x1800+0+0 0mm x 0mm
2880x1800 60.00*+
1600x1200 60.00
1440x1050 60.00
1280x960 60.00
1024x768 60.00
800x600 60.00
640x480 60.00