I disagree as the "yak" has limited area and space and the larger units can get in the way at times. Being a minimalist is always better in a yak. One dunk with a flip over and you can kiss that $1000 unit goodbye! Spend the big bucks if you have them but be forewarned.
From the Humminbird web site:
Humminbird products typically have an IP (Ingress Protection Rating) rating of IPX7. If a product is not IPX7, it will be indicated as such in the product specification section of that model in the Product section of our web site.
This is a tested and rated measurement that indicates the degree of of protection provided against the intrusion of dust, accidental contact and water. Equipment having this rating must be able to withstand submersion in water to a maximum depth of one meter for 30 minutes.
While I do not intend to test their claim Humminbird units, if used correctly, will not stop working in a flip over.
The maximum size of the unit depends on the size of the kayak. I have a Helix 5 on my kayak and where I have it installed increasing the size of the unit would not impact my fishing or paddling.