Vice City is about the same size as Los Santos.
I measured the GTA 6 map without water, the islands, the city of Vice City, and these two peninsulas from end to end, and it's about 19 km. I measured the entire city of LS from GTA 5, and it's 18 km. It will be exactly the same proportions as in GTA 5, except half the map will be rural. Yes, the overall map is larger, but the city itself is exactly the same size as Los Santos in GTA 5.
You can measure just the city without water and Los Santos yourself, and you'll be very surprised. It's important to measure only the city's land area, not just the square.