I was having troubble changing the resolution of DOSBox 6.5 on windowed mode while using D-Fend 2...

It seems that D-Fend's config file was missing "windowresolution=", probablly because of version differences.

So I just added a line "windowresolution=1024x768" under the graphics area of the D-Fend config file like it was in dosbox.conf and now all is peachy.