Playing LA Noire right now and I think that would be a perfect sized map for the city. Obviously have the desert, or woodland or whatever else around it but for the actual city, that would be plenty. Think back to GTA III and VC, you knew those maps inside out. For SA you knew most of it. GTA IV, I'm guessing you only knew bits and pieces of it. The maps can get too big and for me, that takes some of the fun away. Not sure what the size is compared to GTA IV, but for me the LA Noire size would be perfect.
Hmmm, I'm surprised you find it easier to know the LA Noire map than GTAIV... I 100%'d LA Noire, but my knowledge of the city was very limited. Couldn't tell you how to get from one area to the next..
Playing through GTAIV right now, and instantly got my bearings back. I pretty much know the Hove Beach, Dukes, Steinway, Bohan, Northwood and Middle Park areas off by heart. I thought IV was the best map of all the GTA games I've played. The only area I didn't know that well was Alderney, mainly because I never went there except to do missions.
I reckon a good size for Los Santos (the city) would be Algonquin combined with Bohan. Maybe a little bigger (take Charge Island and maybe Steinway as well). Combine that with decent bit of countryside and I'd be happy as a pig in sh1t.