Proper HD export format in Sony Vegas Pro 13 for a decent video size?
I assume you're using the Mainconcept AVC encoder. 3GB for a 1h50m video implies a bitrate of 3.7 mbps. That's the average bitrate you need. Set max bitrate to 5 mbps.

Now, usually I render in two passes. First pass is a high bitrate from Vegas  (20 mbps or more) and then encode with ffmpeg, which is a free command line tool.

Command would be

ffmpeg -i vegas.mp4 -crf 23 -movflags +faststart final.mp4


ffmpeg offers lot more variables to adjust but the above should be enough for most size adjustment. Usually a CRF value between 18-28 (lower is better but bigger) works.