So me and my friend decided to walk from the airport to paleto bay(no sprint!). It took us more than an hour(real time) to reach the middle of the map. We estimate that it takes at least two to two and a half hours to reach paleto if you walk across the highway all the time. So when people say the map is small that`s probably because the drive around it in 10 minutes with their sportscars driving 200 mph/s.
But if you compare GTA V with other open world games like skyrim, fallout or red dead you realise that none of these games have sportscars at all. I doubt it would take you 2,5 hours to walk across skyrim or through the capital wasteland.
Also please note the level of detail gta v has, when you walk across the map with no hud you start to realize how great and believeable this virtual world actually is. And you will realise details that you won't see if you just drive through san andreas. Their are bigger games like just cause 2 I know however their world feels generated and artificial if you walk accross their maps. I recommend you to try it for yourself( no hud,radar walking to san andreas without sprinting)