Attempting to stick with the specific question I am guessing you know where the settings are for the camera to change the photo properties. If so, is this problem only happening on one site? What do you see when you view the photo on your phone?
I'm guessing the problem is the way the site is treating the photo.
I don't think you can adjust the picture size on the Droid Maxx camera. I have not found a picture size setting on this phone, like previous Droid's. But I have not had any issues posting pictures to websites & forums...they always show up full size.
There are plenty of apps on the Play Store that can reduce the resolution and even crop photos after they are taken. No, there is no way to change the resolution with the stock camera app, but the Google camera app, also in the Play Store, gives you a couple of options.