The most reliable way to fix this is by using the by ThirteenAG. 1. Download the Essential Fix
Go to your GTA San Andreas installation directory (e.g., C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto San Andreas ). gta-san-andreas-1080p-resolution-fix-file
The gold standard for this issue is the GTA SA Widescreen Fix. It doesn't just unlock 1080p; it fixes the UI scaling so your map and health bar aren't stretched horizontally. The most reliable way to fix this is