It will be similar in size to Czechia.
Even a little bigger

.

But of course, I believe the developers will pack there as much roads as they can (no more Nevada* road density!). As for the scale, both
ETS and
ATS have 1:20, while 1:35 (scale of the
ATS dork age) proved to be too small

. Since the switch to 1:1 at every city entrance is always done seamlessly (so, no technical difficulties), and since the new DLC is going to be totally separated from the main territory, there is a chance we will see something like 1:10 exclusively for Iceland.
*
BTW, today is a 90th anniversary of Hoover Dam dedication ceremony 
.

Just an easy to miss location in
ATS, but the center of power in the game which is going to celebrate its 15th birthday very soon... nine days after
MLP G4.