Depending on where you intend to place the unit .... measure how far away your eyes will be from the unit, then stand that far away from the store demo models. That'll give you a better idea of how much detail you will be able to see at a glance. Obviously the bigger screens are easier to read, but that comes at a high price that not everyone can afford.
For my money ... a 5" screen is the minimum for a bow unit (if sitting in a pedestal seat). For me, that's almost 5' away from my eyes (unit is mounted on floor of bow). On my Lowrance Elite 5DSI, I've found the blue screen to be the easiest to see, and it shows good detail on that palette color.

There is usually a unit or two for sale on the Personal Classifieds Forum, but if you want new ... price shop the unit you want, then contact Brian@BBGMarine and see if he can beat that price (don't forget to factor in S/H costs, too, if you intend to order online).