@mitu Hi. Yes, I tried that as well. The only way to run DosBox Staging at full-screen is to keep it at texture or texturenb. I tried all the rest, and the result is a business card size display box.
I also added "XINIT:bash /home/pi/RetroPie/roms/pc/+Start\ DOSBox-Staging.sh %ROM%" to my emulators.cfg to see if that would give me the full screen, but it knocks out the keyboard and runs Staging as a slide show.
I'm ok with how it is. I'm going to eventually upgrade to a Pi4. I'm happy just being able to play all these old DOS games again! I had my setup all wrong and now it's completely sorted.