Hi,
If we are talking about the same Slider Widget, then yes, you can resize as you wish. The easiest way to resize is to remove all slides except one and edit that slide as you wish.
When resizing of the slide is done, copy every element in the slide, add a new slide and paste everything in place.
I hope this helps,
If you have any problems in doing this step, feel free to let us know.
Good luck,
Musefree Team