I saw someone asked for a map size comparison between FH5 and GTA V. I threw my current Cities: Skylines map into the mix as well, just for fun
The maps of GTA and C:S are well calibrated to scale. FH5 is an approximation based on the information that it's 50% larger than FH4, for a total area of around 110 square kilometres (including the water), so the scale of that map might not be 100% accurate.
FH5 has a much larger land area than GTA V, although I assume that some of those mountains in the north and south are out of bounds for players.
The volcano in FH5 seems to be similar in size to Mount Chiliad in GTA V.
The airport in FH5 has a runway length of around 1.2 km, compared to just under 1 km in GTA V.
The long bridge in FH5 seems to be 860 meters in length, based on my interpretation of where the bridge starts and ends. By comparison, the suspension bridge in GTA V next to the airport in GTA V is 1 kilometer long.
The largest roundabout in FH5 seems to be similar in size to that roundabout outside the airport in GTA V.
The highway is longer in GTA V, since it loops around the island rather than going from point A to point B. In FH5 it seems to be around the same distance as it would be to drive from Los Santos to that tunnel just after Grapseed in GTA V.
GTA V has a longer coast (since it has water on all sides), although big chunks of those are too steep for driving. Horizon's coastline look more car-friendly.
Los Santos is obviously the biggest city, by quite a margin. Guanajuato seems to similar in size to the combined area of Paleto Bay, Sandy Shores and Grapeseed (excluding the farm area).
The Horizon stadium seems to be around the same size as the container area of the shipping terminal ("Simeon docks") in GTA V.
As for the road layout in general I'm a bit disappointed with FH5, it looks more like a jigsaw puzzle than a realistic road layout. It looks like a hard map to navigate by since most of the roads don't really go anywhere, they just appear to connect at random. Perhaps the intention is to make the highway the most convenient route to take between the major destinations?
This road in particular annoys me
It sure looks fun to drive though