For a Kayak, a 5 or 7" does fine, unless you want live sonar and have the budget for it.
I have a Lowrance Elite Ti7 on my kayak, mounted on the right side rail. Lowrance's user interface is good, but screen is awful compared to Garmin. I believe Lowrance improved their screens since the Elite series.
I have my downscan transducer mounted through a scupper and later added a sidescan unit on a removable arm.
I have a 12 aH lithium battery in a Pelican case that's stored in the hull with disconnects so I can remove and charge it.
If you have a marine store, or Cabelas/Bass Pro near you that has fish finders on display, go play with them in person and see for yourself.
Best of luck choosing your fish finder.