Looking for a HDS 5 with chartplotter but not sure if the screen is big enough for structurescan. The 7" & larger is out of my price range and would like to add structurescan in the future.

I use the Humminbird 798 ci SI that has the same size screen. The smaller screen is a disadvantage if you want to use multiple views, but for single views, they are OK. The two screens I use the most is the Side Imaging only (which is the Structure Scan view shown in the BPS catalog) and the 2D/DI that is positioned one above the other like this--don't know what the HDS5 display setup is---

Larger displays are easier to look at, but the HDS 5 displays StructureScan very well.  The 5s and 7s will only display two viewing panes at a time (8s and 10s will display 4 viewing panes), but you have your choice of displaying two of these four (side scan, downscan, 2D sonar, and map).

Go larger than the 5 inch screen for structure scan