Hey. Recently I purchased Descent 1+2, Redneck Rampage and Shadow Warrior. All games run fine if I don't change any graphic settings but if I change it to D3D, change the resolution etc. all the games launch in a very small screen.

My video card: ATI Radeon HD 4850

Anyone else had this issue? What should I do?

DOSBox only scales as far as the scaler determines, so e.g. a 2x scaler will make a 320x200 game increase to 640x400 or 640x480 (depending on the aspect setting); if you then set the DOSBox resolution higher than 640x480 the output will be surrounded by black.

For most games you want to leave the resolution at original and then use one of the 2x scalers. Everything will fill the screen as expected.

For some games you can use source ports to play them natively under Windows with support for modern resolutions and other features; for Descent 1+2, [url=http://www.jonof.id.au/jfsw]JFSW for Shadow Warrior, etc.

What Arkose said is partially true, but: "if you then set the DOSBox resolution higher than 640x480 the output will be surrounded by black." isn't true.
If you have it set to the correct output mode then it should scale to full screen. Ddraw works the best for me.

I usually have these options set in this way:

fullresolution=0x0

output=ddraw

aspect=true

scaler=normal2x

It is almost surely your monitor settings. While occasionally you may have to tweak DOSBox for some games, I doubt very much that is your issue.

I have an HP w2408h 24" (you almost surely don't), but here is what I did for mine.

Press Menu to bring up the onscreen monitor options.
Select Image Control
Select Custom Scaling

Mine then has three options. Fill to Screen, Fill of Aspect Ratio, and One to One. The DEFAULT is One to One leaving me with the game in a small screen in the center of a black field. Setting it Fill to Aspect will maximize the width (or height) leaving the other dimension from touching the edge of the screen. Fill to Screen does exactly that. Just change it if you need to. Your monitor will be different but the same ideas apply and there are only so many menu options to look at.

When I end the game and go back to my Desktop, the resolution automatically goes back to normal. Problem solved.

I sincerely hope this helps as I know the madness! It took me probably well over a year to figure it out (finally!). Good luck and let me know if it works.