I am playing the Elder Scrolls Arena in fullcreen mode. While I have hq2x, the game is really fullscreened, but when i use hq3x, there are black borders around game, so it is not fully in fullscreen. Any solution?

Can you post the contents of the .conf file you're using? I can diagnose and make suggestions for lines to change.

If your renderer is set to surface or overlay, I recommend changing it to opengl or directdraw.

I'd also recommend changing fullresolution to 0x0.

Head to the game folder and edit dosbox_arena.conf with notepad: *fullresolution should be set to your native screen resolution.

Since I cannot post links for reasons uknown to me, here I post some attached screens of dosbox_arena.conf.

I have Direct3D renderer.
My fullresolution equals nothing. Changing it to my native screen resolution or 0x0 does not help, possibly it makes the border even wider.
Aspect is already true.
Thanks for quick responding.

If fullresolution is blank then it's likely set to "original" which if you are using an LCD is likely not ideal. Set it to 0x0 to use your desktop resolution. Then you can play around with DOSBox software scalers or let your video card or monitor scale the image, you're always going to have black bars when attempting to view DOS games correctly on a LCD and you should be happy to see them since that means you're likely viewing the image correctly (or rather more correctly than without).

When fullresolution equals nothing / "original", there is black border, that I was talking about.
When fullresolution equals "0x0" / "desktop", there is black border, that is even wider thank border mentioned above.

By the way, what is the difference between "fullresolution" and "windowresolution"?

Check what the scaling settings are in your graphics card properties.

Fullresolution= full screen
Windowresolution= Window

Typically you don't set the Windowresolution to same resolution as your desktop since you want to multitask.

Or you can just set it to 0x0, which uses the desktop resolution. :P Why didn't you try any of the renderers I suggested? Unless you're using a special build of DOSBox, Direct3D isn't even an option which might mean it's defaulting to surface which can't scale and would give exactly the result you described (higher resolution = bigger borders).

I find OpenGL to give the best results but you should try directdraw as well and see which is best for you.

What graphics card are you using?

In the NVidia/AMD control panel you should have an option of changing "scaling". If you switch it to "maintain aspect ratio" it should work for all Dosbox and non-Dosbox games.

Attached is for NVidia.

Make sure "Perform scaling on" is set to GPU.

I have already tried same things you have set in you NVIDIA control panel. That does not work.
More I am trying, more is border wider.

Wait...

If you're using a 1920x1200 monitor, then it's normal that you get balck borders on the side. The game was meant for 4:3 resolution.

My resolution is 1920x1080.
And everything is going good, when using 2x scaler (hq2x) and when using fullresolution= (without nothing).

The problem comes from the hq3x, I have the same problem on my own dosbox install when I put it on. Somehow, activating it decreases the game's resolution, hence the black border around it. I don't know how to keep the conf's resolution (be it windowed or fullscreen) when the hq3x is activated (as well as the 2xsai, super2xsai, supereagle, etc.). I didn't have any problem however with the advmame or advinterp ones. Either use these ones, or switch from direct3d to opengl which smoothes the image a bit anyway, without the need for additional filters.

Besides you could increase the number of cycles a bit to have a smoother game : 30000 might be better than 20000.
And setting on the double buffering might be a bit better too.

When you say arena is running fullscreen with no borders check your monitor OSD and see what resolutions it's running in.