while compiling the source..I notice you can compile using different features.
i uploaded 3 so far....
basiliskIIGS--->sdl/gtk/fbdev...not sure what the issues is with it but the screen is all screwed up.... ./BasiliskII --screen dga/0/0 start in fullscreen.

basiliskGTK--->works nicely....

and BasiliskIISDL-->works nicely...does 720x480 which is the best screen res I've use so far on the emu. And is also the version that requires command line parameters. ./BasiliskII --rom /lo/ca/tion/of/rom --disk sys/tem/dsk --disk hdd/dsk

compiling basiliskIISDL and BasiliskIIGS with the --enable-XF86-dga does the fullscreen...but when it goes into fullscreen its starts up, but the screen is black. ./BasiliskII --screen dga/0/0

another issue notice with the dga/0/0 when it does go into full screen sometimes require a reboot to get out.....I advice you to install MPV, so the need to turn the device off won't be needed.

Unix:
The "video mode" is one of the following:
win/<width>/<height>
Color display in an X11 window of the given size. There are several
resolutions and color depths available. The set of color depths
depends on the capabilities of the X11 server, the operating system,
and Basilisk II compile-time options, but 1 bit and the default depth
of the X11 screen should always be available.
dga/<width>/<height>
[if Basilisk II was configured with --enable-xf86-dga]
Full-screen display using the XFree86 DGA extension. The color depth
(8/15/24 bit) depends on the depth of the underlying X11 screen.
"width" and "height" specify the maximum width/height to use.
Saying "dga/0/0" means "complete screen".
dga/<frame buffer name>
[if Basilisk II was configured with --enable-fbdev-dga]
Full-screen display using the frame buffer device /dev/fb. The color
depth (8/15/24 bit) depends on the depth of the underlying X11 screen.
The "frame buffer name" is looked up in the "fbdevices" file (whose
path can be specified with the "fbdevicefile" prefs item) to determine
certain characteristics of the device (doing a "ls -l /dev/fb" should
tell you what your frame buffer name is).

Best result so far is to run BasiliskIISDL and when the emu starts go to control panel and monitor settings and do 720x480.

Now I guess hacking away at the source would be a good thing....but don't expect miracles from me, and If I do walk on water then be happy...