Check the standard dosbox.conf. It describes two options, fullfixed and fullresolution in the sdl section.

I like to use:
fullfixed=true
fullresolution=640x480

Kippesoep: You should leave fullfixed=false and let the game decide which resolutoin to use.

Just a thought: if I create a custom 640x350 resolution via PowerStrip, will DOSBox be able to detect it as a usable screen mode for the EGA games that need it (instead of falling back to the typical 640x400 mode and displaying black bars at top and bottom)?

I think if SDL.DLL doesn't support it, you are out of luck.

Kaminari: If you have an nVidia card you don't even need PowerStrip. Geforce drivers allow you to add almost any new resolution.

When I tried playing with rare resolutions i had to use multiplied resoluions (for example 1280x700) and I had to use appropriate dosbox scaler to force it.
This way I was even able to play Flashback (256x224) if fullscreen without any black boarders. I just added 1024x896 resoluion and configured dosbox with:
fullresolution=1024x896
output=openglnb
It worked perfectly even on an LCD monitor (with analog cable) 😀

(By The Way, Streatching 640x350 games to 4:3 screen will make them rather ugly.)