I think that,while it was of good size,the countryside in GTA 5 was poorly designed.Most of it was useless mountains,and whatever flat ground was left was covered in man-made objects (roads, trailer parks, big airfield in the middle of the desert, random buildings every few dozen yards alongside every road...),which makes it looks weird.
Here's how I would design the countryside:
-it should be from the same size as the city to twice the size of the city
-it should have a few towns far away from each other (not too close like Sandy Shores, Grapeseed and Harmony)
-towns should be designed like real towns (like Palomino Creek or Fort Carson from GTA SA,with a few dozen streets, a residential "neighborhood" with houses, middle of the town with a police station, a hospital, some stores, a local diner and a local bar) not just a few houses and stores around a single road like Harmony
-country roads should feel more empty (have a lone motel, diner or a gas station every few miles,not every few dozen yards)
-countryside should have a large forest or grasslands or desert or swamps or whatever,and feel empty too (when you're in the middle of a desert or a forest you shouldn't see random signs, fences and other man-made objects everywhere you look)
-if there's any mountains,there should be only one or two,they shouldn't take up 70% of the countryside
The city could be designed better too.For example,the middle class suburbs and ghettos could be larger in comparison to the rich neighborhoods.