You can easily increase the resolution of the slides your using by going into Inspecter, click on the very first button which looks like a plain piece of paper.
Then you should see a option which says "Slide Size" and from there you can pick whatever resolution that you want, if you want to resize more than one slide then simply click all the slides and follow the same instructions.
Keynote is a great piece of software, its far superior than Microsoft Powerpoint any day of the week, but its sometimes hard to find even the simpliest things. Hope this helped!