FWIW I used a Garmin 12 as my prime nav unit for a while until I tripped over the Lowrance on the bits & pieces section here.
I agree 5" is probably the lower limit for a plotter , but when I was using the 12 I built up a set of waypoints & routes that I could use to monitor my position relative to the solid stuff. I then set the XTC error to 1/4 mile, and switched it to "motoway mode" - as long as I stuck within the edges I knew I was clear of the rocks. Also that is a nice visiual on a 1" LD B&W screen while bouncing along at 20 knots!
Granted it did involve a bit of pre planning, but I stil l carry it as backup as the waypoints are still valid & it runs for 24 hrs on a set of AAs.......
Re. screen size on a plotter I just set mine to overlay speed & depth at top left & right, in "large font" and left the fishfinder graphic display on another menu, theory being All I really need to know is how much water under the keel.
You can pick up a used Garmin 72 for sub £100. If you really want mapswhile you wait for the plotter there's always the 76 or equivalent....