You most likely saw my pic of my 800 and 770 side by side. I do rationalize the 10 might be too big. I’m growing happier and happier with the 800’s extras.
I have used the 7" Rand McNally GPS for 2 1/2 years and like the size. My trainer had the 8" version and I didn't think the slightly larger size was worth the extra $100, especially when there was more to go wrong with the larger version that included tablet features.
As an alternative, before you lay down points or dollars, try Hammer on your smartphone or notepad for 30 days. It's free and I believe it will do as much or more than a high buck GPS will do. I've been using it for over 2 years and don't see an advantage with any brand over this.
OTR800 low on the windshield flush with the dash as low as it goes, offset the the right of the driver but not quite mid point. Perfect size for that position.
I'd say 5''. Plenty big, and less expensive. I use Rand McNallys...I don't necessarily recommend them , I had to return/replace a few units over the years due to charging problems, screen freezing and the like but the last one I got has lasted 3/4 years and I'd expect the same from Garmin or any other brand name.
I have the 800. Here's my set up. Since I had to use my cellphone to take the pic, it isn't in it. But, you can see it's mount so mentally picture a cellphone just above my Garmin