A quick crash course on "fish finders"
2d sonar and "down looking" (down imaging/ down view) imaging helps you to see what is beneath you.
"Side seeing" (side imaging/side view) helps you to see what is off to the side of your boat. With this you see below and to the side a certain distance up to and over 150 feet to the side.
I would not go smaller than 7 inches as far a screen size goes. What I would do is read, read, and read more on what each feature can do and how it could or could not help you. The more you inform yourself the better decision you'll make. I can tell you my opinion and everyone else can too but they're only opinions.
We are all ready and willing to spend your money for you, but if you don't know what they do it won't do you any good. Hope this helps, good luck, and don't hesitate to ask anymore questions.