Wild Guess: Monitor adjustmen neccessary. Try the settings your display offers, h-/v-size, pincushion and trapez correction and that stuff. If the picture is almost the expected size, but distorted and pushed up a bit, that may help. It may be a totally different problem, however.

Try also a different output method (surface, overlay, OpenGL) and update video drivers if needed.

I agree with Moe - it's probably that DOSBox is using a low-resolution fullscreen mode that you haven't used before, in which case you need to adjust your monitor to make it look right.

it could be a refresh rate glitch too ....dont know

Good point