If you open you camera app and swipe to the right a Photos Options menu should show. The one related to picture size only lets you choose between widescreen and standard. It doesn't let you pick a file size or lower resolution. In order to change the picture size, you my need an app like "Resize Me!" on Google Play. When you get to that app, other similar apps will be listed to the side. I suggest experimenting with one or more of the apps to see which app or reduction setting gives you the best result. You will lose some quality but it may not be noticeable.
There my be camera apps that allow size setting not available in your standard phone app but the few that I have used did not have that feature.