When I'm in a group up to 3 players (which I usually am because I mainly play arena), I have 200% scale, but when I enter a battleground with 10 to 40 players (or even a group of 4 or more players, really), the frames are too large for that group. However, I can set them to 100% scale and they'll look perfect and still readable, but just show everybody. This is because I have carefully made sure my Grid2 profile still looks great at 100% as well as 200%. I just like it larger when there are only 1 to 3 players.

Manually setting the scale to 100% every time I'm in a group of 4 or more players and then back up to 200% when I'm done is a bit of a burden. It would be nice if I could set a scale by a group size threshold, with the scale changing to a configured value when the player count increases above the configured threshold.

My small addon works perfectly for my needs and I've documented it here in case others want to use it. Thanks for considering the feature request. I'm closing this as a result of the discussion and my testing here.

You can enable themes (General/General Tab) and then configure a different theme for each type of content (solo/arena/party/raid) or by raid size 5,10,15, up to 40, or by spec or role or class.
On each theme you can configure a different appearance, including scale, sizes,, different indicators, layouts etc.
Or you can goto layouts/by Raid size tab, and set a diferent width/height for the frames depending of raid size 5,10,15, etc.
Sorry, Im not going to add thresholds not multiple of 5 for raid size, to much changes needed for a feature almost nobody is going to use.

I wrote a small addon for myself that does what I want with hard-coded scale values. It works well in my testing with battlegrounds! Here's the toc file, Grid2AutoScaler.toc:

## Interface: 110100, 40402, 30403, 20505, 11507
## Title: Grid2AutoScaler
## Notes: Scales Grid2 depending on group member count
## Author: BujuArena
## Dependencies: Grid2
## LoadWith: Grid2

Grid2AutoScaler.lua

Here's the content of Grid2AutoScaler.lua:

hooksecurefunc(Grid2, "UpdateRoster", function(self, ...)
	if GetNumGroupMembers() <= 3 then
		if Grid2Layout.db.profile.ScaleSize < 2 then
			Grid2Layout.db.profile.ScaleSize = 2
			Grid2Layout:RefreshLayout()
		end
	else
		if Grid2Layout.db.profile.ScaleSize > 1 then
			Grid2Layout.db.profile.ScaleSize = 1
			Grid2Layout:RefreshLayout()
		end
	end
end)

License: CC0