How do i make Elder scrolls battlespire full screen ? Right now its just showing up as a small screen. Please help

Try editing the dosbox_battlespire.conf file in your Battlespire folder (using Notepad or any other text editor): Change the line "output=overlay" to "output=ddraw" or "output=opengl" (without quotes). This should make the game screen bigger, but will leave some black space on the right and left side of your monitor screen, and you will see a 1024x768 display stretched to fit your monitor screen as good as possible.
You can also experiment with other values of the "scaler" parameter, or try "aspect=true". You may find parameters which make the game look slightly better, but the two problems I mentioned above are not going to go away.
Don't forget to create a copy of the .conf file before you edit it - just in case you don't like a change, but don't remember what exactly you've changed.

Alt+Enter switches between windowed and full screen modes in DOSBox (in which Battlespire is running).

Yes (by the way: the GOG version starts in fullscreen by default) - but I think the question was about enlarging the "game screen", the screen area the game is displayed in. If your monitor has a resolution of more than 1024x768, the game screen fills only an area which reflects the proportion of 1024x768 and your monitor resolution, the rest is black. The black frame doesn't look nice on large high-resolution monitors, and makes playing awkward because it's difficult to recognise details on the small game screen.
Changing the output parameter of DOSBox in the way I described adjusts the height of the game screen to the monitor height and enlarges the width of the game screen accordingly. This will still leave black borders to the right and left of the game screen, as the aspect ratio isn't changed. And the enlargement has its impact on the graphics quality, as the resolution is still 1024x768. Both effects get worse with increasing monitor size.

Fine - as long as you can live with the black borders to the left and right of the game display (on a non-4:3, wide-screen monitor). Have fun!