You should ask this question in the proper forum, but you can see the game fullscreen I think. Mess around with the dosbox.conf file like this, not sure if this will work:

fullscreen=false
fulldouble=false
fullresolution=Original (I think this is the one you need)
windowresolution=Original

Haven't used DosBox for a while but I did always get it to fill the screen.

maybe i'm using the wrong resolution?
fullscreen=true
fulldouble=true
fullresolution=Original
windowresolution=Original
output=overlay
because it use some of screen, but not all of screen.
if i use 'overlay' the image is showed on from top of screen(black border on bottom). if i use other output i will get the blck border on top.
what you can tell me more?

using:
fullscreen=true
fulldouble=false
fullresolution=1366X768
windowresolution=original
output=overlay
the image height is full, but the weight isn't 🙁
i'm testing now

fullresolution=desktop
aspect=false
output=(ANYTHING THAT IS NOT SURFACE)

the problem remains:
fullscreen=true
fulldouble=false
fullresolution=desktop
windowresolution=desktop
aspect=false
output=overlay
autolock=true
sensitivity=100
waitonerror=true
priority=higher,normal
mapperfile=mapper-0.74.map
usescancodes=true
the width resolution isn't all screen

Which monitor? How is it connected? Which operating system?

You need to set "output" to some value with scaling, e.g.:

surfacenp -- near-perfect scaling via oversampling;
surfacenb and openglnb -- nearest-neighbor scaling.

Also, "fullresolution" is not only true or false. You can give a fixed resolution here to optimize display and scaling.

i'm using Windows 7 on laptop.
i had tested several possible output without sucess... i don't think we can combine them... but tell me more

Either be happy with a distorted image with a small black bar or a fairly accurate image with large black bars. Pick one.

i'm sorry, but these black bars can't be avoided?
https://imgur.com/a/FW75Gjb
at least i can use the height entirely after several searchs

They can if you tell DOSBox to ignore aspect ratio. But trust me, that is not want you want to do...

please, let me try.
please, tell me what is the config name

I am also trying to do this. I tried so many different options and I can't get rid of the side bars either.
I do not want to use "original" as resolution because it makes the monitor go lower resolution to accomplish the fullscreen, even though it does get rid of the bars.
Staying at "0x0" or "desktop" keeps my monitor at my native resolution but introduces the bars.
Anyone?