Installing this game on Windows 10 64 bit , also needs Dos Box. Starting the game might already be set up for Dos Box. The game shows a very small screen. The game default resolution is 640/480. All you have to do is set your resolution to a very low size. The lowest I could go is 800/600 and the game screen is much larger.

You don't need to change your desktop resolution. Scaling settings can be found in you graphics control panel.

Indeed.

Course setting it in dosbox settings, look for the .conf file, probably dosbox_gamename.conf

Search for scaler=, set to normal2x, normal3x or higher if supported. (640x480 x3 = 1920x1440)

Probably don't use full screen.

But you can set it to always start in full screen, with or without black bars.

Yep, this is definitely the easiest way to handle upscaling.

Here's a good article on GPU scaling methods and how to enable scaling on different GPU brands, for anyone who isn't familiar with how to do this:
https://themonitormonitor.com/what-is-gpu-scaling-how-to-enable-or-disable-it/

And that's the issue... the OP has more black bars than game screen because no scaling is used.
"full screen" actually just means "non-windowed"; scaling decides how that screen is filled - controlled by software (dosbox here) or by GPU/Monitor: fill, (keep) aspect ratio + fill, or integer scaling.

Video Card Amd Radeon 6900 series
Element brand monitor 800/600-1920/1080 HDMI.

Tried all suggestions and nothing seems to work except setting video to 800/600.
At least game screen is almost full size and is more manageable.

Did you install the Radeon driver (from AMD's website)?
Is it a laptop or desktop?
To configure scaling: https://www.amd.com/en/support/kb/faq/dh-019

Yes, and desktop

did an update request and windows determined that I have the best video card driver at this time.

Don't use the driver provided by Windows Update... install the driver that can be found on the AMD website.
The windows update driver is just meant for general usage, not for games.

installed latest driver. still have to use 800/600 here is the game screen.
There's less than an inch black all around sides.

Found a post in the Battlespire posts.

in the game directory\dosbox_battlepire.conf

it was set for output=overlay.

I changed it to output=ddraw

now it's a nice big game screen without fooling with the monitor resolution. However, the small black boardering can be elimented: Also with the monitor resolution set to 800/600, there is no boarder at all. It's a complete game screen top to bottom and left to right. W O W